2025 AMC 8 第 24 题

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所有题目均经美国数学协会(MAA)官方合法授权使用。

24.

在梯形 ABCDABCD 中,角 BB 和角 CC 都是 6060^\circ,且 AB=DCAB = DC。所有边长都是正整数,梯形 ABCDABCD 的周长为 3030 个单位。有多少个互不全等的梯形满足这些条件?

In trapezoid ABCD,ABCD, angles BB and CC measure 6060^\circ and AB=DC.AB = DC. The side lengths are all positive integers and the perimeter of ABCDABCD is 3030 units. How many non-congruent trapezoids satisfy all of these conditions?

00

11

22

33

44

答案:E
知识点:梯形等边三角形丢番图方程
难度评级:1840
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文字解答:

AA 作一条平行于 CDCD 的线,并观察到有些长度会自动相等:

这是因为 AEB=DCB=60\angle AEB = \angle DCB = 60^\circ,所以三角形 ABEABE 是等边三角形。所有标为 xx 的线段总是相等。

另外,ADCEADCE 是平行四边形(不一定是菱形),所以其余标为 yy 的线段彼此相等,但不一定等于 xx

周长为 3x+2y3x + 2y,题目要求它等于 3030

问题就变成数方程 3x+2y=303x + 2y = 30 有多少组正整数解。

考虑正整数 xx 时,只有偶数值会给出整数 yy,因为 303x30 - 3x 必须是偶数 2y2y

而当 x=10x = 10 时,满足方程的 yyy=303×102=0y = \frac{30 - 3 \times 10}{2} = 0,不是正整数。

所以可行的 xx 是小于 1010 的正偶数,即 {2,4,6,8}\{2, 4, 6, 8\}。共有 44 种,答案是 E

Draw a line through AA parallel to line CD,CD, and observe that some lengths are automatically equal to each other:

That is because AEB=DCB=60,\angle AEB = \angle DCB = 60^\circ, and so triangle ABEABE is equilateral. All lengths labeled xx are always equal.

Also, ADCEADCE is a parallelogram (not necessarily a rhombus), so the remaining lengths labeled yy are always equal to each other, but not necessarily equal to x.x.

The perimeter is 3x+2y,3x + 2y, but it is also supposed to be 30.30.

The problem then amounts to counting how many positive integer solutions there are to the equation 3x+2y=30.3x + 2y = 30.

As we consider positive integers for x,x, observe that it is precisely the even integers which give an integer solution for y,y, because 303x30 - 3x is the even number 2y.2y.

And, once we get to x=10,x = 10, the yy that satisfies the equation is y=303×102=0,y = \frac{30 - 3 \times 10}{2} = 0, which is not a positive integer.

So, the values that work for xx are the positive even integers less than 10,10, or {2,4,6,8}.\{2, 4, 6, 8\}. There are 44 options, which gives the answer of E.

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