2010 AMC 12B 第 19 题

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19.

Raiders 队和 Wildcats 队的一场高中篮球赛在第一节结束时打平。Raiders 队四节的得分构成递增等比数列,Wildcats 队四节的得分构成递增等差数列。第四节结束时,Raiders 队以一分获胜。两队得分都不超过 100100 分。上半场两队总共得了多少分?

A high school basketball game between the Raiders and the Wildcats was tied at the end of the first quarter. The number of points scored by the Raiders in each of the four quarters formed an increasing geometric sequence, and the number of points scored by the Wildcats in each of the four quarters formed an increasing arithmetic sequence. At the end of the fourth quarter, the Raiders had won by one point. Neither team scored more than 100100 points. What was the total number of points scored by the two teams in the first half?

3030

3131

3232

3333

3434

答案:E
知识点:等比数列等差数列极限情形界定
难度评级:2180
解答:

设 Raiders 四节的得分依次为 a,ar,ar2,ar3a, ar, ar^2, ar^3(递增等比数列,r>1r\gt1),Wildcats 的得分依次为 a,a+d,a+2d,a+3da, a+d, a+2d, a+3d(递增等差数列),两队第一节都得 a.a.

r=m/nr=m/n 写成最简分数,其中 m>n.m\gt n. 因为 ar3ar^3 是整数,所以 n3a;n^3\mid a;a=An3.a=A n^3. Raiders 的总分为 R=A(n3+mn2+m2n+m3). R=A(n^3+mn^2+m^2n+m^3). 由于 m3<R100,m^3\lt R\le100, 可知 m4.m\le4. 数对 (4,3)(4,3) 已使括号内的和达到 175,175, 所以互质数对 (m,n)(m,n) 只可能是 (2,1),(3,1),(3,2),(4,1).(2,1),(3,1),(3,2),(4,1).

相应的 R/AR/A 分别为 15,40,65,85.15,40,65,85. 对于 (4,1)(4,1)(3,2),(3,2), 上界迫使 A=1,A=1, 但由 R1=4a+6dR-1=4a+6d 得到的 d.d. 不是整数。对于 (3,1),(3,1), 会得到 36A1=6d,36A-1=6d, 这在模 6.6. 意义下不可能。

(m,n)=(2,1),(m,n)=(2,1), 时,R=15AR=15Aa=A,a=A, 所以 11A1=6d.11A-1=6d. 因而 A5(mod6),A\equiv5\pmod6, 再由 R100R\le100A=5.A=5. 此时 d=9,d=9, Raiders 的得分是 5,10,20,405,10,20,40,Wildcats 的得分是 5,14,23,32.5,14,23,32. Raiders 以 757574.74. 获胜。

上半场总分为 (5+10)+(5+14)=34.(5+10)+(5+14)=34.

因此,正确答案是 E

Let the Raiders score a,ar,ar2,ar3a, ar, ar^2, ar^3 (increasing geometric, r>1r\gt1) and the Wildcats a,a+d,a+2d,a+3da, a+d, a+2d, a+3d (increasing arithmetic), tied in the first quarter at a.a.

Write r=m/nr=m/n in lowest terms, with m>n.m\gt n. Since ar3ar^3 is an integer, n3a;n^3\mid a; put a=An3.a=A n^3. The Raiders' total is R=A(n3+mn2+m2n+m3). R=A(n^3+mn^2+m^2n+m^3). Since m3<R100,m^3\lt R\le100, we have m4.m\le4. The pair (4,3)(4,3) already makes the parenthesized sum 175,175, so the only possible coprime pairs (m,n)(m,n) are (2,1),(3,1),(3,2),(4,1).(2,1),(3,1),(3,2),(4,1).

The corresponding base values of R/AR/A are 15,40,65,85.15,40,65,85. For (4,1)(4,1) and (3,2),(3,2), the bound forces A=1,A=1, but R1=4a+6dR-1=4a+6d gives a nonintegral d.d. For (3,1),(3,1), it would give 36A1=6d,36A-1=6d, which is impossible modulo 6.6.

For (m,n)=(2,1),(m,n)=(2,1), we have R=15AR=15A and a=A,a=A, so 11A1=6d.11A-1=6d. Thus A5(mod6),A\equiv5\pmod6, and R100R\le100 forces A=5.A=5. Then d=9,d=9, giving Raiders scores 5,10,20,405,10,20,40 and Wildcats scores 5,14,23,32.5,14,23,32. The Raiders win 7575 to 74.74.

The first-half total is (5+10)+(5+14)=34.(5+10)+(5+14)=34.

Thus, the correct answer is E.

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