2012 AMC 10A 真题

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1.

Cagney 每 2020 秒给一个纸杯蛋糕涂糖霜,Lacey 每 3030 秒给一个纸杯蛋糕涂糖霜。两人一起工作,55 分钟内可以给多少个纸杯蛋糕涂糖霜?

Cagney can frost a cupcake every 2020 seconds and Lacey can frost a cupcake every 3030 seconds. Working together, how many cupcakes can they frost in 55 minutes?

1010

1515

2020

2525

3030

答案:D
知识点:速率单位换算

难度评级:560

解答:

Cagney 每分钟完成 60÷20=360 \div 20 = 3 个,Lacey 每分钟完成 60÷30=260 \div 30 = 2 个。

55 分钟内,他们能完成 5(2+3)=55=25. 5(2 + 3) = 5 \cdot 5 = 25.

所以正确答案是 D

Cagney can make 60÷20=360 \div 20 = 3 cupcakes per minute, and Lacey can make 60÷30=2.60 \div 30 = 2.

In 55 minutes, they can make 5(2+3)=55=25. 5(2 + 3) = 5 \cdot 5 = 25.

Thus, D is the correct answer.

2.

一个边长为 88 的正方形被切成两块全等的长方形。每个长方形的边长是多少?

A square with side length 88 is cut in half, creating two congruent rectangles. What are the dimensions of one of these rectangles?

2 by 42 \text{ by } 4

2 by 62 \text{ by } 6

2 by 82 \text{ by } 8

4 by 44 \text{ by } 4

4 by 84 \text{ by } 8

答案:E

难度评级:450

解答:

注意其中一条边仍与原正方形的边长相同。

因此一个尺寸是 88,另一个尺寸是原边长的一半,即 44

所以正确答案是 E

Note that the one of the sides remains the same as the original square.

This means that one dimension is 8.8. The other dimension is the original side cut in half, which is 4.4.

Thus, E is the correct answer.

3.

一只小虫在数轴上从 2-2 爬到 6-6,再爬到 55。它总共爬了多少距离?

A bug crawls along a number line, starting at 2.-2. It crawls to 6,-6, then turns around and crawls to 5.5. How many units does the bug crawl altogether?

99

1111

1313

1414

1515

答案:E
知识点:绝对值

难度评级:560

解答:

2-26-6 的距离为 2(6)=4=4 |-2 - (-6)| = |-4| = 4 然后,从 6-655 的距离为 65=11=11 |-6 - 5| = |-11| = 11

总距离为 4+11=15. 4 + 11 = 15.

所以正确答案是 E

It crawls a distance of 2(6)=4=4 |-2 - (-6)| = |-4| = 4 when it moves from 2-2 to 6.-6. It then travels a distance of 65=11=11 |-6 - 5| = |-11| = 11 as it moves from 6-6 to 5.5.

The total distance is then 4+11=15. 4 + 11 = 15.

Thus, E is the correct answer.

4.

已知 ABC=24\angle ABC = 24^\circABD=20\angle ABD = 20^\circ CBD\angle CBD 的最小可能度数是多少?

Let ABC=24\angle ABC = 24^\circ and ABD=20.\angle ABD = 20^\circ . What is the smallest possible degree measure for CBD?\angle CBD?

00

22

44

66

1212

答案:C
知识点:导角最优化

难度评级:770

解答:

注意两个角都共用射线 ABAB。为了使所求角最小,让 DD 位于 AACC 之间。

这样就有 CBD=ABCABD=4. \begin{aligned} \angle CBD &= \angle ABC - \angle ABD \\ &= 4^{\circ}. \end{aligned}

所以正确答案是 C

Note that both of the angles share the ray AB.AB. To minimize the desired degree, we want DD to be between AA and C.C.

This would make CBD=ABCABD=4. \begin{aligned} \angle CBD &= \angle ABC - \angle ABD \\ &= 4^{\circ}. \end{aligned}

Thus, C is the correct answer.

5.

某收容所有 100100 只成年猫,其中一半是母猫。成年母猫中有一半各带着一窝小猫,每窝平均有 44 只小猫。这个收容所共有多少只猫和小猫?

Last year 100100 adult cats, half of whom were female, were brought into the Smallville Animal Shelter. Half of the adult female cats were accompanied by a litter of kittens. The average number of kittens per litter was 4.4. What was the total number of cats and kittens received by the shelter last year?

150150

200200

250250

300300

400400

答案:B
知识点:分数平均数

难度评级:870

解答:

成年母猫有 100÷2=50100 \div 2 = 50 只,其中带小猫的有 50÷2=2550 \div 2 = 25 只。

因为平均每窝有 44 只小猫,所以小猫总数为 425=100. 4 \cdot 25 = 100.

因此成年猫和小猫的总数为 100+100=200. 100 + 100 = 200.

所以正确答案是 B

We have that there are 100÷2=50100 \div 2 = 50 female cats. We then have that 50÷2=2550 \div 2 = 25 cats that have kittens.

Since the average number of kittens per litter is 4,4, the total number of kittens is 425=100. 4 \cdot 25 = 100.

The total number of cats and kittens is then 100+100=200. 100 + 100 = 200.

Thus, B is the correct answer.

6.

两个正数的乘积为 99。其中一个数的倒数是另一个数倒数的 44 倍。这两个数的和是多少?

The product of two positive numbers is 9.9. The reciprocal of one of these numbers is 44 times the reciprocal of the other number. What is the sum of the two numbers?

103\dfrac{10}{3}

203\dfrac{20}{3}

77

152\dfrac{15}{2}

88

答案:D

难度评级:1070

解答:

设两个数为 xxyy,满足 xy=9 and 1x=4y. xy = 9 \text{ and } \dfrac{1}{x} = \dfrac{4}{y}.

由乘积条件得 y=9x y = \dfrac{9}{x} 代入倒数条件,得到 1x=4x9. \dfrac{1}{x} = \dfrac{4x}{9}. 因为 xx 为正,所以 x=32, x = \dfrac{3}{2},

于是另一个数为 y=9÷32=6. y = 9 \div \dfrac{3}{2} = 6.

两数之和为 6+32=152. 6 + \dfrac{3}{2} = \dfrac{15}{2}.

所以正确答案是 D

Let the two numbers be xx and yy such that xy=9 and 1x=4y. xy = 9 \text{ and } \dfrac{1}{x} = \dfrac{4}{y}.

We get that y=9x y = \dfrac{9}{x} 1x=4x9. \dfrac{1}{x} = \dfrac{4x}{9}. x=32, x = \dfrac{3}{2}, since xx is positive.

Then y=9÷32=6. y = 9 \div \dfrac{3}{2} = 6.

The desired sum is then 6+32=152. 6 + \dfrac{3}{2} = \dfrac{15}{2}.

Thus, D is the correct answer.

7.

一个袋子里有若干弹珠,其中 35\dfrac{3}{5} 是蓝色的,其余是红色的。现在把红色弹珠的数量加倍,而蓝色弹珠数量不变。此时红色弹珠占全部弹珠的几分之几?

In a bag of marbles, 35\dfrac{3}{5} of the marbles are blue and the rest are red. If the number of red marbles is doubled and the number of blue marbles stays the same, what fraction of the marbles will be red?

25\dfrac{2}{5}

37\dfrac{3}{7}

47\dfrac{4}{7}

35\dfrac{3}{5}

45\dfrac{4}{5}

答案:C
知识点:比与比例分数

难度评级:870

解答:

不妨设原来共有 55 颗弹珠,其中蓝色有 33 颗,红色有 22 颗。

红色数量加倍后有 44 颗红弹珠,因此红色所占比例为 44+3=47. \dfrac{4}{4 + 3} = \dfrac{4}{7}.

所以正确答案是 C

WLOG, let the number of marbles in the bag be 5.5. Since we only care about ratios, we can do this. Then there are 33 blue marbles and 22 red marbles.

Doubling the red marbles gives us 44 of them. Then the fraction of red marbles is 44+3=47. \dfrac{4}{4 + 3} = \dfrac{4}{7}.

Thus, C is the correct answer.

8.

三个非负整数两两相加得到的和分别为 12,1712, 171919。这三个数中的中间那个数是多少?

The sums of three whole numbers taken in pairs are 12,17,12, 17, and 19.19. What is the middle number?

44

55

66

77

88

答案:D
知识点:方程组

难度评级:1020

解答:

设三个数为 a,b,ca, b, c,且 a<b<ca \lt b \lt c。由于三个和都不同,所以这三个数也互不相等。

题意给出 a+b=12,a+c=17, a + b = 12, a + c = 17, 以及 b+c=19. b + c = 19.

把三式相加,得到 2(a+b+c)=48. 2(a + b + c) = 48.

因此 a+b+c=24, a + b + c = 24, 再减去 a+c=17, a + c = 17, 得到 b=7b = 7

所以正确答案是 D

Let the three numbers be a,b,ca, b, c where a<b<c.a \lt b \lt c. None of them are equal, since all three sums are different.

Then a+b=12,a+c=17, a + b = 12, a + c = 17, and b+c=19. b + c = 19.

Adding all three equations together gives us 2(a+b+c)=48. 2(a + b + c) = 48.

Then a+b+c=24, a + b + c = 24, from which we can subtract a+c=17, a + c = 17, to get b=7.b = 7.

Thus, D is the correct answer.

9.

一枚偶数骰子的六个面上只标有偶数,其中 224466 各出现两次。另一枚奇数骰子的六个面上只标有奇数,其中 113355 各出现两次。同时掷这两枚骰子,顶面点数之和为 77 的概率是多少?

A pair of six-sided dice are labeled so that one die has only even numbers (two each of 2,2, 4,4, and 66), and the other die has only odd numbers (two of each 1,1, 3,3, and 55). The pair of dice is rolled. What is the probability that the sum of the numbers on the tops of the two dice is 7?7?

16\dfrac{1}{6}

15\dfrac{1}{5}

14\dfrac{1}{4}

13\dfrac{1}{3}

12\dfrac{1}{2}

答案:D

难度评级:960

解答:

和为 77 的点数对是 (2,5),(4,3), and (6,1). (2, 5), (4, 3), \text{ and } (6, 1).

得到其中任意一个指定点数对的概率为 1313=19 \dfrac{1}{3} \cdot \dfrac{1}{3} = \dfrac{1}{9}

共有 33 个这样的点数对,所以和为 77 的总概率为 319=13. 3 \cdot \dfrac{1}{9} = \dfrac{1}{3}.

所以正确答案是 D

The pairs of numbers that sum to 77 are (2,5),(4,3), and (6,1). (2, 5), (4, 3), \text{ and } (6, 1).

There is a 1313=19 \dfrac{1}{3} \cdot \dfrac{1}{3} = \dfrac{1}{9} chance that we get any of these pairs.

There are 33 pairs, which means that the total probability that the rolls sum to 77 is 319=13. 3 \cdot \dfrac{1}{9} = \dfrac{1}{3}.

Thus, D is the correct answer.

10.

一个圆被分成 1212 个扇形。每个扇形的圆心角度数都是整数,并且这十二个角的度数构成一个等差数列。最小扇形角的最小可能度数是多少?

Mary divides a circle into 1212 sectors. The central angles of these sectors, measured in degrees, are all integers and they form an arithmetic sequence. What is the degree measure of the smallest possible sector angle?

55

66

88

1010

1212

答案:C

难度评级:1420

解答:

设最小扇形角为 aa,等差数列的公差为 dd

这些角的总和为 122(a+a+11d)=12a+66d \dfrac{12}{2}(a + a + 11d) = 12a + 66d

总和是 360360,所以 12a+66d=360 12a + 66d = 360 2a+11d=60. 2a + 11d = 60.

要使 aa 尽可能小,就要让 dd 尽可能大。若 d=5d = 5,则 aa 不是整数;因此 d=4d = 4,且 a=8a = 8

所以正确答案是 C

Let aa be the smallest possible sector angle and dd be the difference in the arithmetic sequence.

Then we have that 122(a+a+11d)=12a+66d \dfrac{12}{2}(a + a + 11d) = 12a + 66d is the sum of the arithmetic sequence.

We have that this sums to 360,360, so 12a+66d=360 12a + 66d = 360 2a+11d=60. 2a + 11d = 60.

We want to minimize a,a, so we maximize d.d. If d=5,d = 5, then aa is not an integer, so d=4d = 4 and a=8.a = 8.

Thus, C is the correct answer.

11.

两个圆外切,圆心分别为 AABB,半径分别为 5533。一条公共外切线与射线 ABAB 相交于点 CC。求 BCBC 的长度。

Externally tangent circles with centers at points AA and BB have radii of lengths 55 and 3,3, respectively. A line externally tangent to both circles intersects ray ABAB at point C.C. What is BC?BC?

44

4.84.8

10.210.2

1212

14.414.4

答案:D

难度评级:1420

解答:

xxBCBC。注意 CEB\triangle CEBCDA\triangle CDA 相似。

于是 x3=8+x5. \dfrac{x}{3} = \dfrac{8 + x}{5}. 交叉相乘得到 5x=24+3x 5x = 24 + 3x x=12. x = 12.

所以正确答案是 D

Let xx be BC.BC. Note that CEB\triangle CEB and CDA\triangle CDA are similar due to angle-angle (tangent lines are perpendicular to radii).

Then x3=8+x5. \dfrac{x}{3} = \dfrac{8 + x}{5}. Cross-multiplying gives us 5x=24+3x 5x = 24 + 3x x=12. x = 12.

Thus, D is the correct answer.

12.

根据闰年规则,年份能被 400400 整除就是闰年,例如 20002000 年;或者能被 44 整除但不能被 100100 整除也是闰年,例如 20122012 年。Charles Dickens 诞辰 200200 周年的庆祝日是 20122012 年二月 77 日,星期二。他出生那天是星期几?

A year is a leap year if and only if the year number is divisible by 400400 (such as 20002000) or is divisible by 44 but not 100100 (such as 20122012). The 200200th anniversary of the birth of novelist Charles Dickens was celebrated on February 7,7, 2012,2012, a Tuesday. On what day of the week was Dickens born?

星期五

Friday

星期六

Saturday

星期日

Sunday

星期一

Monday

星期二

Tuesday

答案:A

难度评级:1540

解答:

平年有 365365 天,向未来移动一年,星期几会向后移动一天,因为 365=527+1. 365 = 52 \cdot 7 + 1.

闰年有额外的一天,所以星期几会向后移动两天。

1812181220122012 之间通常每 44 年有一个闰年,但要去掉 19001900,所以共有 4949 个闰年。

因此,往回推 200200 年需要往回移动 200+49=249=357+4 200 + 49 = 249 = 35 \cdot 7 + 4 天,也就是星期几往回移动 44 天。

星期二往前推四天是星期五,也就是 18121812 年二月 77 日。

所以正确答案是 A

On a typical year with 365365 days, moving one year into the future moves the day of the week forward one since 365=527+1. 365 = 52 \cdot 7 + 1.

On a leap year however, the day of the week gets moved forward twice since there is an extra day.

Fifty of the years between 20122012 and 18121812 are multiples of 4,4, but we have to discard 1900,1900, which leaves us with 4949 leap years.

Therefore, moving back 200200 years means we have to go back 200+49=249=357+4 200 + 49 = 249 = 35 \cdot 7 + 4 days, which means we move back 44 days in the week.

This takes us back to Friday, which is the day that corresponds to February 7,7, 1812.1812.

Thus, A is the correct answer.

13.

1122334455 按某种顺序排列。先求前两个数的平均数,再把这个平均数与第三个数求平均数,继续这样依次与第四个、第五个数求平均数。最终结果的最大可能值与最小可能值之差是多少?

An iterative average of the numbers 1,1, 2,2, 3,3, 4,4, and 55 is computed the following way. Arrange the five numbers in some order. Find the mean of the first two numbers, then find the mean of that with the third number, then the mean of that with the fourth number, and finally the mean of that with the fifth number. What is the difference between the largest and smallest possible values that can be obtained using this procedure?

3116\dfrac{31}{16}

22

178\dfrac{17}{8}

33

6516\dfrac{65}{16}

答案:C

难度评级:1600

解答:

设排列顺序为 a,b,c,d,e. a, b, c, d, e.

则迭代平均数为 a+b2+c2+d2+e2 \dfrac{\dfrac{\dfrac{\dfrac{a + b}{2} + c}{2} + d}{2} + e}{2}=a+b+2c+4d+8e16. = \dfrac{a + b + 2c + 4d + 8e}{16}.

要使结果最小,应按 5,4,3,2,1, 5, 4, 3, 2, 1, 排列,得到 5+4+6+8+816=3116. \dfrac{5 + 4 + 6 + 8 + 8}{16} = \dfrac{31}{16}.

要使结果最大,则反过来排列,得到 1+2+6+16+4016=6516. \dfrac{1 + 2 + 6 + 16 + 40}{16} = \dfrac{65}{16}.

两者之差为 65163116=3416=178. \dfrac{65}{16} - \dfrac{31}{16} = \dfrac{34}{16} = \dfrac{17}{8}.

所以正确答案是 C

Let the order of the numbers be a,b,c,d,e. a, b, c, d, e.

Then the iterative average is a+b2+c2+d2+e2 \dfrac{\dfrac{\dfrac{\dfrac{a + b}{2} + c}{2} + d}{2} + e}{2}=a+b+2c+4d+8e16. = \dfrac{a + b + 2c + 4d + 8e}{16}.

To minimize this, we make the order 5,4,3,2,1, 5, 4, 3, 2, 1, which gives us a sum of 5+4+6+8+816=3116. \dfrac{5 + 4 + 6 + 8 + 8}{16} = \dfrac{31}{16}.

To maximize it, we have to reverse this order to get an average of 1+2+6+16+4016=6516. \dfrac{1 + 2 + 6 + 16 + 40}{16} = \dfrac{65}{16}.

The difference between these is 65163116=3416=178. \dfrac{65}{16} - \dfrac{31}{16} = \dfrac{34}{16} = \dfrac{17}{8}.

Thus, C is the correct answer.

14.

一个每边有 3131 个方格的棋盘按黑白相间染色,并且四个角都是黑色。黑色方格有多少个?

Chubby makes nonstandard checkerboards that have 3131 squares on each side. The checkerboards have a black square in every corner and alternate red and black squares along every row and column. How many black squares are there on such a checkerboard?

480480

481481

482482

483483

484484

答案:B

难度评级:1020

解答:

共有 1515 行各含 1515 个黑格,另有 1616 行各含 1616 个黑格。

第一行有 1616 个黑格,其余各行按 1515 个与 1616 个交替。

因此共有 1616 行各含 1616 个黑格,其余各行含 1515 个黑格。

因此黑格总数为 152+162=481. 15^2 + 16^2 = 481.

所以正确答案是 B

Note that there are 1515 rows with 1515 black tiles and 1616 rows with 1616 black tiles.

This can be seen by observing that the first row has 1616 black tiles, and all the other rows alternate with 1515 and 1616 tiles.

Then, due to the alternating pattern, there will be a total of 1616 rows with 1616 tiles, and the other rows have 1515 tiles.

The total number of black squares is then 152+162=481. 15^2 + 16^2 = 481.

Thus, B is the correct answer.

15.

下图由三个单位正方形和两条线段组成。求 ABC\triangle ABC 的面积。

Three unit squares and two line segments connecting two pairs of vertices are shown. What is the area of ABC?\triangle ABC?

16\dfrac16

15\dfrac15

29\dfrac29

13\dfrac13

24\dfrac{\sqrt{2}}{4}

答案:B

难度评级:1480

解答:

可以用坐标几何求两条直线的交点。

AA 为原点,B=(1,0)B = (1, 0)。经过 AA 的直线斜率为 12-\dfrac{1}{2},方程为 y=12x. y = -\dfrac{1}{2}x. 经过 BB 的直线斜率为 22yy 轴截距为 2-2,所以方程为 y=2x2. y = 2x - 2.

联立两式,得到 2x2=12x 2x - 2 = -\dfrac{1}{2}x x=45. x = \dfrac{4}{5}.

因此点 CCyy 坐标为 1245=25. -\dfrac{1}{2} \cdot \dfrac{4}{5} = -\dfrac{2}{5}.

三角形 ABCABC 的面积为 12125=15. \dfrac{1}{2} \cdot 1 \cdot \dfrac{2}{5} = \dfrac{1}{5}.

所以正确答案是 B

We can use coordinate geometry to figure out where the intersection of the two lines occurs.

Let AA be the origin and B=(1,0).B = (1, 0). Then the slope of the line through AA is 12,-\dfrac{1}{2}, which makes the equation of the line y=12x. y = -\dfrac{1}{2}x. The slope of the line through BB is 2.2. The yy-intercept is 2.-2. This makes the equation of this line y=2x2. y = 2x - 2.

Equating the equations, we get 2x2=12x 2x - 2 = -\dfrac{1}{2}x x=45. x = \dfrac{4}{5}.

This makes the yy-coordinate of CC 1245=25. -\dfrac{1}{2} \cdot \dfrac{4}{5} = -\dfrac{2}{5}.

The area of triangle ABCABC is then 12125=15. \dfrac{1}{2} \cdot 1 \cdot \dfrac{2}{5} = \dfrac{1}{5}.

Thus, B is the correct answer.

16.

三名跑步者同时从一条 500500 米环形跑道的同一点出发,沿顺时针方向跑。他们的速度分别为每秒 4.4,4.84.4, 4.85.05.0 米。三人第一次再次同时相遇时,他们已经跑了多少秒?

Three runners start running simultaneously from the same point on a 500500-meter circular track. They each run clockwise around the course maintaining constant speeds of 4.4,4.8,4.4, 4.8, and 5.05.0 meters per second. The runners stop once they are all together again somewhere on the circular course. How many seconds do the runners run?

1,0001,000

1,2501,250

2,5002,500

5,0005,000

10,00010,000

答案:C
解答:

先求速度为 4.84.8 米/秒的跑者追上最慢跑者所需的时间。

必须有 4.8x4.4x=500 4.8x - 4.4x = 500 x=1250, x = 1250, 其中 xx 是较快跑者追上另一人的时间。

注意 4.41250=55004.4 \cdot 1250 = 5500,所以这两名跑者每次相遇都在起点。

现在要求最小的 tt,使 tt12501250 的倍数,并且最快的跑者也回到起点。

12501250 秒,最快的跑者跑 12505=62501250 \cdot 5 = 6250 米;到 25002500 秒时,他跑了 1250012500 米,正好是整数圈,其余条件也满足。

所以正确答案是 C

Let us find the amount of time that it takes for the runner running at 4.84.8 meters per second to lap the slowest person.

We must have that 4.8x4.4x=500 4.8x - 4.4x = 500 x=1250, x = 1250, where xx is the amount of time it takes for the faster runner to lap the other.

Note that 4.41250=5500,4.4 \cdot 1250 = 5500, which means that these two runners always intersect at the starting line.

We now have to find the least time, t,t, such that tt is a multiple of 12501250 and the fastest runner ends up at the starting line.

Every 12501250 seconds, the fastest runner runs 12505=62501250 \cdot 5 = 6250 meters. Then in 25002500 seconds, the fastest runner runs 1250012500 meters, which is a whole number of laps.

Thus, C is the correct answer.

17.

正整数 aabb 互质,满足 a>b>0a > b > 0,且 a3b3(ab)3=733.\dfrac{a^3-b^3}{(a-b)^3} = \dfrac{73}{3}.aba-b

Let aa and bb be relatively prime positive integers with a>b>0a > b > 0 and a3b3(ab)3=733.\dfrac{a^3-b^3}{(a-b)^3} = \dfrac{73}{3}. What is ab?a-b?

11

22

33

44

55

答案:C

难度评级:1670

解答:

先因式分解 a3b3=(ab)(a2+ab+b2). a^3 - b^3 = (a - b)(a^2 + ab + b^2). 消去这个因子后得到 a2+ab+b2a22ab+b2=733. \dfrac{a^2 + ab + b^2}{a^2 - 2ab + b^2} = \dfrac{73}{3}.

交叉相乘并整理,得到 70a2149ab+70b2=0. 70a^2 - 149ab + 70b^2 = 0. 因为 b0b \neq 0,可同除以 b2b^2,得到 70(ab)2149ab+70=0. 70\left(\dfrac{a}{b}\right)^2 - 149\dfrac{a}{b} + 70 = 0.

用二次公式,并注意 a>ba \gt b,可得 ab=107\dfrac{a}{b} = \dfrac{10}{7}

由于 aabb 互质,所以 a=10a = 10b=7b = 7,差为 107=310 - 7 = 3

所以正确答案是 C

Recall that we can factor a3b3=(ab)(a2+ab+b2). a^3 - b^3 = (a - b)(a^2 + ab + b^2). Canceling out this factor gives us that a2+ab+b2a22ab+b2=733. \dfrac{a^2 + ab + b^2}{a^2 - 2ab + b^2} = \dfrac{73}{3}.

Cross-multiplying and rearranging gives us 70a2149ab+70b2=0. 70a^2 - 149ab + 70b^2 = 0. Since b0,b \neq 0, we can divide through by b2b^2 to get 70(ab)2149ab+70=0. 70\left(\dfrac{a}{b}\right)^2 - 149\dfrac{a}{b} + 70 = 0.

Applying the quadratic formula and noting that a>ba \gt b gives us that ab=107.\dfrac{a}{b} = \dfrac{10}{7}.

Since aa and bb are relatively prime, we have that a=10a = 10 and b=7.b = 7. Their difference is 107=3.10 - 7 = 3.

Thus, C is the correct answer.

18.

图中的闭合曲线由 99 段全等圆弧组成,每段圆弧长为 2π3\dfrac{2\pi}{3},每段对应圆的圆心都是边长为 22 的正六边形的某个顶点。该闭合曲线围成的面积是多少?

The closed curve in the figure is made up of 99 congruent circular arcs each of length 2π3,\dfrac{2\pi}{3}, where each of the centers of the corresponding circles is among the vertices of a regular hexagon of side 2.2. What is the area enclosed by the curve?

2π+62\pi+6

2π+432\pi+4\sqrt{3}

3π+43\pi+4

2π+33+22\pi+3\sqrt{3}+2

π+63\pi+6\sqrt{3}

答案:E

难度评级:1930

解答:

如图,把区域与正六边形比较;六边形外侧共有 33 个角为 240240^{\circ} 的扇形。

这说明它们合起来形成 22 个半径为 11 的完整圆。现在还要计算六边形内部且在曲线内部的面积。

这个面积可以用正六边形面积减去曲线外侧的扇形面积得到。

共有三个 13\dfrac{1}{3} 圆合起来形成一个完整圆。正六边形面积为 32223=63. \dfrac{3}{2} \cdot 2^2 \cdot \sqrt{3} = 6\sqrt{3}.

所求面积为 2π+(63π)=π+63. 2\pi + (6\sqrt{3} - \pi) = \pi + 6\sqrt{3}.

所以正确答案是 E

Note that the region enclosed by the curve but outside the hexagon consists of 33 sectors with angle 240.240^{\circ}.

This means that together they form 22 whole circles with radius 1.1. Now to find the area of the region inside both the hexagon and the curve.

This area can be found by finding the area of the hexagon and subtracting out the areas of the sectors outside the curve.

There are three 13\dfrac{1}{3} circles that together form a whole circle. The area of the hexagon can be given by 32223=63. \dfrac{3}{2} \cdot 2^2 \cdot \sqrt{3} = 6\sqrt{3}.

The desired area is then 2π+(63π)=π+63. 2\pi + (6\sqrt{3} - \pi) = \pi + 6\sqrt{3}.

Thus, E is the correct answer.

19.

油漆工 Paula 和她的两名助手各自以固定但不同的速度刷漆。他们总是上午 8:008:00 开始工作,并且三人每天午餐休息的时间相同。

星期一,三人一起刷完了一栋房子的 50%50\%,并在下午 4:004:00 停工。星期二,Paula 不在,两名助手只刷完了房子的 24%24\%,并在下午 2:122:12 停工。星期三,Paula 独自工作,到晚上 7:127:12 完成整栋房子。

每天的午餐休息是多少分钟?

Paula the painter and her two helpers each paint at constant, but different, rates. They always start at 8:008:00 AM, and all three always take the same amount of time to eat lunch.

On Monday the three of them painted 50%50\% of a house, quitting at 4:004:00 PM. On Tuesday, when Paula wasn't there, the two helpers painted only 24%24\% of the house and quit at 2:122:12 PM. On Wednesday Paula worked by herself and finished the house by working until 7:127:12 P.M.

How long, in minutes, was each day's lunch break?

3030

3636

4242

4848

6060

答案:D
知识点:速率方程组

难度评级:2060

解答:

设午餐休息为 mm 分钟,Paula 的工作速度为每分钟 pp 个百分点,两名助手的合计速度为每分钟 hh 个百分点。

星期一给出 (p+h)(480m)=50(p+h)(480-m)=50。星期二给出 h(372m)=24h(372-m)=24。星期三剩余工作量为百分之 2626,所以 p(672m)=26p(672-m)=26

由后两式相加再减去第一式,得到 108h192p=0108h-192p=0,所以 h=169ph=\dfrac{16}{9}p

代入星期一的方程得 259p(480m)=50\dfrac{25}{9}p(480-m)=50,而星期三方程为 p(672m)=26p(672-m)=26。联立解得 m=48m=48

所以正确答案是 D

Let the lunch break be mm minutes, Paula's rate be pp percent per minute, and the helpers' combined rate be hh percent per minute.

Monday gives (p+h)(480m)=50(p+h)(480-m)=50. Tuesday gives h(372m)=24h(372-m)=24. Since the remaining work on Wednesday was 2626 percent, Wednesday gives p(672m)=26p(672-m)=26.

Adding the Tuesday and Wednesday equations and subtracting the Monday equation gives 108h192p=0108h-192p=0, so h=169ph=\dfrac{16}{9}p.

Substituting into Monday gives 259p(480m)=50\dfrac{25}{9}p(480-m)=50, while Wednesday gives p(672m)=26p(672-m)=26. Solving these two equations gives m=48m=48.

Thus, D is the correct answer.

20.

一个 3×33 \times 3 网格的 99 个小方格独立随机染成黑色或白色。

将整个网格顺时针旋转 9090^{\circ} 后,把每个现在位于原来黑格位置上的白格染成黑色,其余方格不变。最终整个网格全为黑色的概率是多少?

A 3×33 \times 3 square is partitioned into 99 unit squares. Each unit square is painted either white or black with each color being equally likely, chosen independently and at random.

The square is then rotated 9090^{\circ} clockwise about its center, and every white square in a position formerly occupied by a black square is painted black. The colors of all other squares are left unchanged. What is the probability the grid is now entirely black?

49512\dfrac{49}{512}

764\dfrac{7}{64}

1211024\dfrac{121}{1024}

81512\dfrac{81}{512}

932\dfrac{9}{32}

答案:A

难度评级:1980

解答:

中心格必须一开始就是黑色,贡献概率 12\dfrac12。四个角在旋转下形成一个循环,四个边中点也形成另一个相同的循环。

对一个 44 循环,除非有一个白格被旋转到原本也是白格的位置,否则最后四个位置都会变黑。成功的初始颜色序列为 BBBBBBBBBBBWBBBW 的四种旋转,以及 BWBWBWBW 的两种旋转,共 77 种,占 1616 种。

边中点循环有相同的独立计数。因此概率为 12(716)2=49512\dfrac12\left(\dfrac7{16}\right)^2=\dfrac{49}{512}

所以正确答案是 A

The center square must initially be black, contributing probability 12\dfrac12. The four corner squares form one cycle under the rotation, and the four edge-middle squares form another identical cycle.

For one 44-cycle, the final four positions are all black unless a white square is rotated into a position that was also white. The successful initial colorings are BBBBBBBB, the four rotations of BBBWBBBW, and the two rotations of BWBWBWBW, for 77 colorings out of 1616.

The same count applies to the edge-middle cycle, independently. Therefore the probability is 12(716)2=49512\dfrac12\left(\dfrac7{16}\right)^2=\dfrac{49}{512}.

Thus, A is the correct answer.

21.

设点 A=(0,0,0),A = (0 ,0 ,0), B=(1,0,0),B = (1, 0, 0), C=(0,2,0),C = (0, 2, 0), D=(0,0,3).D = (0, 0, 3).E,E, F,F, G,G,HH 分别是线段 BD, AB, AC,\overline{BD},\text{ } \overline{AB}, \text{ } \overline {AC},DC\overline{DC} 的中点。求四边形 EFGHEFGH 的面积。

Let points A=(0,0,0),A = (0 ,0 ,0), B=(1,0,0),B = (1, 0, 0), C=(0,2,0),C = (0, 2, 0), D=(0,0,3).D = (0, 0, 3). Points E,E, F,F, G,G, and HH are midpoints of line segments BD, AB, AC,\overline{BD},\text{ } \overline{AB}, \text{ } \overline {AC}, and DC\overline{DC} respectively. What is the area of EFGH?EFGH?

2\sqrt{2}

253\dfrac{2\sqrt{5}}{3}

354\dfrac{3\sqrt{5}}{4}

3\sqrt{3}

273\dfrac{2\sqrt{7}}{3}

答案:C

难度评级:1730

解答:

注意 EF=12ADEF = \dfrac{1}{2}AD,因为它是 ABD\triangle ABD 的中位线。同理,HG=12ADHG = \dfrac{1}{2}AD,且 FG=12BCFG = \dfrac{1}{2}BC

又因为 EF\overline{EF}HG\overline{HG} 垂直于 xyxy 平面,所以它们垂直于 FG\overline{FG}EH\overline{EH}

因此 EFGHEFGH 是长方形,且 EF=HGEF = HG。有 EF=123=32EF = \dfrac{1}{2} \cdot 3 = \dfrac{3}{2}

还可得 FG=1212+22=52. FG = \dfrac{1}{2} \sqrt{1^2 + 2^2} = \dfrac{\sqrt{5}}{2}.

所以 EFGHEFGH 的面积为 EFFG=3252=354. EF \cdot FG = \dfrac{3}{2} \cdot \dfrac{\sqrt{5}}{2} = \dfrac{3\sqrt{5}}{4}.

所以正确答案是 C

Note that EF=12ADEF = \dfrac{1}{2}AD since it is a midsegment of ABD.\triangle ABD. Similarly, HG=12ADHG = \dfrac{1}{2}AD and FG=12BC.FG = \dfrac{1}{2}BC.

We also have that EF\overline{EF} and HG\overline{HG} are perpendicular to the xyxy-plane, which means that they are perpendicular to FG\overline{FG} and EH.\overline{EH}.

This tells us that EFGHEFGH is rectangle since EF=HG.EF = HG. We have EF=123=32.EF = \dfrac{1}{2} \cdot 3 = \dfrac{3}{2}.

We also have that FG=1212+22=52. FG = \dfrac{1}{2} \sqrt{1^2 + 2^2} = \dfrac{\sqrt{5}}{2}.

The area of EFGHEFGH is then EFFG=3252=354. EF \cdot FG = \dfrac{3}{2} \cdot \dfrac{\sqrt{5}}{2} = \dfrac{3\sqrt{5}}{4}.

Thus, C is the correct answer.

22.

mm 个正奇数的和比前 nn 个正偶数的和多 212212。所有可能的 nn 的和是多少?

The sum of the first mm positive odd integers is 212212 more than the sum of the first nn positive even integers. What is the sum of all possible values of n?n?

255255

256256

257257

258258

259259

答案:A

难度评级:2260

解答:

mm 个正奇数之和为 m2m^2,前 nn 个正偶数之和为 n(n+1)n(n+1)。因此 m2=n(n+1)+212m^2=n(n+1)+212

把它看成关于 nn 的二次方程,判别式为 14(212m2)=4m28471-4(212-m^2)=4m^2-847,必须是一个奇平方数。设 p2=4m2847p^2=4m^2-847,则 (2m+p)(2mp)=847(2m+p)(2m-p)=847

847847 的正因数配对为 8471847\cdot11217121\cdot7771177\cdot11。它们分别给出 p=423,57,33p=423,57,33

因为 n=1+p2n=\dfrac{-1+p}{2},所以 nn 的可能值为 211,28,16211,28,16,它们的和为 255255

所以正确答案是 A

The first mm positive odd integers sum to m2m^2, and the first nn positive even integers sum to n(n+1)n(n+1). Thus m2=n(n+1)+212m^2=n(n+1)+212.

As a quadratic in nn, this has discriminant 14(212m2)=4m28471-4(212-m^2)=4m^2-847, which must be an odd square. Let p2=4m2847p^2=4m^2-847. Then (2m+p)(2mp)=847(2m+p)(2m-p)=847.

The positive factor pairs of 847847 are 8471847\cdot1, 1217121\cdot7, and 771177\cdot11. They give p=423,57,33p=423,57,33, respectively.

Because n=1+p2n=\dfrac{-1+p}{2}, the possible values of nn are 211,28,16211,28,16. Their sum is 255255.

Thus, A is the correct answer.

23.

Adam、Benin、Chiang、Deshawn、Esther 和 Fiona 都有网络账号。在这六个人之间,有些但不是所有人互为好友;他们没有小组外的好友。若每个人的好友数都相同,则可能的好友关系图共有多少种?

Adam, Benin, Chiang, Deshawn, Esther, and Fiona have internet accounts. Some, but not all, of them are internet friends with each other, and none of them has an internet friend outside this group. Each of them has the same number of internet friends. In how many different ways can this happen?

6060

170170

290290

320320

660660

答案:B

难度评级:2200

解答:

按每个人的好友数分类。这个数可以从 1144,因为好友图既不是空图也不是完全图。

注意,11 个好友和 22 个好友的情况分别与 44 个好友和 33 个好友的情况通过补图对应,因为确定谁是好友也就确定了谁不是好友。

情况一:每个人有 11 个好友。

这意味着 66 个人必须分成 33 对,每一对中的两个人互为好友。

第一个人的好友有 55 种选择,剩下 44 个人。

下一位未配对的人的好友有 33 种选择,剩下的 22 个人就被迫成为一对好友。

因此这种情况有 35=153 \cdot 5 = 15 种可能。

情况二:每个人有 22 个好友。

这种情况有两种可能。可能是两个三人组,每个三人组内部人人互为好友。

这一种中,选择第一个三人组有 (63)=20\binom{6}{3} = 20 种方法。因为两个三人组可以互换,所以要除以 22,得到 20÷2=1020 \div 2 = 10 种配置。

第二种可能是好友关系形成一个六边形,每个顶点上的人与相邻两人互为好友。

第一个人可以放在六边形的任意位置。选择与这个人相邻的两个人有 (52)=10\binom{5}{2} = 10 种方法。

最后 33 个人可以用 3!=63! = 6 种方式放入剩余位置。因此这种情况共有 10+106=70 10 + 10 \cdot 6 = 70 种配置。

所以安排总数为 2(15+70)=170. 2(15 + 70) = 170.

所以正确答案是 B

We case on the value of friends that each person has. This value ranges from 11 to 44, since the graph is neither empty nor complete.

Note that the cases for 11 and 22 friends correspond with the case for 44 and 33 friends, since choosing who are friends determines who are not friends.

Case 1: everyone has 11 friend

This means that the 66 people must split up into 33 pairs where the people in each pair are friends.

There are 55 choices for the friend for the first person. This leaves 44 people remaining.

There are then 33 choices for the friend of the next unpaired person. The remaining 22 people are then forced to be friends.

Therefore, there are 35=153 \cdot 5 = 15 possibilities for this case.

Case 2: everyone has 22 friends

There are two possibilities for this case. There could be two triples where everyone in a triple is friends with each other.

For this possibility, there are (63)=20\binom{6}{3} = 20 ways to choose the people in the first triple. We have to divide by 22 since we can swap the pairs. This gives us 20÷2=1020 \div 2 = 10 configurations.

The second possibility is if the friends form a hexagon where the person at each vertex is friends with the adjacent people.

The first person can be placed anywhere on the hexagon. There are (52)=10\binom{5}{2} = 10 ways to choose the people adjacent to this person.

The final 33 people can be placed in 3!=63! = 6 ways in the remaining spots. This case then has a total number of 10+106=70 10 + 10 \cdot 6 = 70 configurations.

The total number of arrangements is then 2(15+70)=170. 2(15 + 70) = 170.

Thus, B is the correct answer.

24.

正整数 a,a, b,b,cc 满足 aa\ge bb\ge cc,且 a2b2c2+ab=2011a^2-b^2-c^2+ab=2011 以及 a2+3b2+3c23ab2ac2bca^2+3b^2+3c^2-3ab-2ac-2bc=1997.=-1997.aa

Let a,a, b,b, and cc be positive integers with aa\ge bb\ge cc such that a2b2c2+ab=2011a^2-b^2-c^2+ab=2011 and a2+3b2+3c23ab2ac2bca^2+3b^2+3c^2-3ab-2ac-2bc=1997.=-1997. What is a?a?

249249

250250

251251

252252

253253

答案:E

难度评级:2350

解答:

两个方程相加,得到 2(a2+b2+c2)2(ab+ac+bc) 2(a^2 + b^2 + c^2) - 2(ab + ac + bc)=14. = 14.

把左边分组并因式分解,得到 (ab)2+(ac)2+(bc)2 (a - b)^2 + (a - c)^2 + (b - c)^2=14. = 14.

左边每一项都是非负整数的平方。三个平方数和为 1414 的唯一方式是 9,49, 411

在三组差中,aca - c 最大,所以 ac=3a - c = 3

还不能确定另外两个差分别对应哪个平方。先试 ab=1a - b = 1bc=2b - c = 2

把这些值代入第一个方程,得到 a2(a1)2(a3)2 a^2 - (a - 1)^2 - (a - 3)^2 +a(a1)=2011. + a(a - 1) = 2011. 化简得 7a=20217a = 2021。由于 20212021 不能被 77 整除,所以应有 ab=2a - b = 2bc=1b - c = 1

再次代入并求解,得到 a=253a = 253

所以正确答案是 E

Adding together the equations gives us 2(a2+b2+c2)2(ab+ac+bc) 2(a^2 + b^2 + c^2) - 2(ab + ac + bc)=14. = 14.

We can group terms and factor this to get (ab)2+(ac)2+(bc)2 (a - b)^2 + (a - c)^2 + (b - c)^2=14. = 14.

Note that every term on the left hand side is a nonnegative square integer. The only triple of squares that add to 1414 is 9,4,9, 4, and 1.1.

We have that aca - c is the biggest difference among the three pairs. Therefore, ac=3.a - c = 3.

We cannot discern which of the other terms we can match with the other squares. Let us try ab=1a - b = 1 and bc=2.b - c = 2.

Plugging in these values into the first equation gives us a2(a1)2(a3)2 a^2 - (a - 1)^2 - (a - 3)^2 +a(a1)=2011. + a(a - 1) = 2011. Simplifying yields 7a=2021.7a = 2021. Since 20212021 is not divisible by 7,7, we have that ab=2a - b = 2 and bc=1.b - c = 1.

Plugging these in again and solving gives us a=253.a = 253.

Thus, E is the correct answer.

25.

实数 x,x, y,y,zz 独立且均匀地从区间 [0,n][0,n] 中选取,其中 nn 是正整数。使得 x,x, y,y,zz 中任意两个数之间的距离都大于一的概率超过 12\dfrac{1}{2}。满足条件的最小 nn 是多少?

Real numbers x,x, y,y, and zz are chosen independently and at random from the interval [0,n][0,n] for some positive integer n.n. The probability that no two of x,x, y,y, and zz are within 1 unit of each other is greater than 12.\dfrac{1}{2}. What is the smallest possible value of n?n?

77

88

99

1010

1111

答案:D

难度评级:2460

解答:

这个问题可以看作几何概率问题,把区间看成坐标轴上的范围。

不妨先考虑 nxyz0. n \geq x \geq y \geq z \geq 0.

满足这个顺序限制的点 (x,y,z)(x, y, z) 构成一个四面体。

这个四面体的高为 nn,底面积为 n22\dfrac{n^2}{2},所以体积为 13n22n=n36. \dfrac{1}{3} \cdot \dfrac{n^2}{2} \cdot n = \dfrac{n^3}{6}.

现在加入题目中的限制。需要找出满足 xy,xz,yz1. |x - y|, |x - z|, |y - z| \geq 1. 的区域。

根据已经规定的顺序,这些不等式可化为 xy1 and yz1. x - y \geq 1 \text{ and } y - z \geq 1.

这两个限制形成如下图所示的另一个四面体。

注意,在新的四面体中,所有线性尺寸都减少了 22。因此高为 n2n - 2,底面积为 (n2)22\dfrac{(n - 2)^2}{2}

因此体积为 13(n2)22(n2) \dfrac{1}{3} \cdot \dfrac{(n - 2)^2}{2} \cdot (n - 2) =(n2)36. = \dfrac{(n - 2)^3}{6}.

所求概率为 (n2)36÷n36=(n2)3n3. \dfrac{(n - 2)^3}{6} \div \dfrac{n^3}{6} = \dfrac{(n - 2)^3}{n^3}.

逐一检查选项可知,使这个比例大于 12\dfrac{1}{2} 的最小值是 1010

所以正确答案是 D

This problem lends itself to geometric probability since we can view the interval as a range on an axis.

WLOG, let nxyz0. n \geq x \geq y \geq z \geq 0.

Then we have that the points (x,y,z)(x, y, z) which satisfy this restriction form a tetrahedron.

The height of this tetrahedron is n,n, and the base has an area of n22.\dfrac{n^2}{2}. This makes the volume 13n22n=n36. \dfrac{1}{3} \cdot \dfrac{n^2}{2} \cdot n = \dfrac{n^3}{6}.

Now we have to apply the restrictions from the problem statement. We need to find the region where xy,xz,yz1. |x - y|, |x - z|, |y - z| \geq 1.

From our ordering condition that we imposed, these inequalities reduce to xy1 and yz1. x - y \geq 1 \text{ and } y - z \geq 1.

These two restrictions form another tetrahedron as shown below.

Note that in the new tetrahedron, all the dimensions have been reduced by 2.2. This makes the height n2n - 2 and the base (n2)22.\dfrac{(n - 2)^2}{2}.

The volume is then 13(n2)22(n2) \dfrac{1}{3} \cdot \dfrac{(n - 2)^2}{2} \cdot (n - 2)=(n2)36. = \dfrac{(n - 2)^3}{6}.

The desired probability is then (n2)36÷n36=(n2)3n3. \dfrac{(n - 2)^3}{6} \div \dfrac{n^3}{6} = \dfrac{(n - 2)^3}{n^3}.

Plugging in all the answer choices, we get that the smallest value such that this fraction is greater than 12\dfrac{1}{2} is 10.10.

Thus, D is the correct answer.