2015 AMC 12B 第 23 题

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23.

一个长方体的尺寸为 a×b×ca \times b \times c, 其中 aabb, 和 cc 是整数,且 1abc1 \le a \le b \le c。 这个长方体的体积和表面积在数值上相等。有多少个有序三元组 (a,b,c)(a, b, c) 是可能的?

A rectangular box measures a×b×c,a \times b \times c, where a,a, b,b, and cc are integers and 1abc.1 \le a \le b \le c. The volume and the surface area of the box are numerically equal. How many ordered triples (a,b,c)(a, b, c) are possible?

44

1010

1212

2121

2626

答案:B
知识点:丢番图方程因式分解分类讨论
难度评级:2340
解答:

体积和表面积在数值上相等,意味着 abc=2(ab+bc+ca).abc=2(ab+bc+ca). 两边除以 abcabc1=2/a+2/b+2/c6/a,1=2/a+2/b+2/c\le6/a,所以 a6.a\le6. 情形 a=1a=1a=2a=2 都没有正数解。当 a2,a\ne2, 时,令 u=(a2)b2a,v=(a2)c2a. \begin{gathered} u=(a-2)b-2a,\\ v=(a-2)c-2a. \end{gathered} 原方程可因式分解为 uv=4a2.uv=4a^2.

a=3,a=3, 时,(b6)(c6)=36(b-6)(c-6)=36 给出 (b,c)=(7,42),(b,c)=(7,42), (8,24),(8,24), (9,18),(9,18), (10,15),(10,15), (12,12).(12,12).a=4,a=4, 时,(b4)(c4)=16(b-4)(c-4)=16 给出 (5,20),(6,12),(8,8).(5,20),(6,12),(8,8).a=5,a=5, 时,两个因数模 33 的同余条件只留下有效数对 (b,c)=(5,10),(b,c)=(5,10),a=6,a=6, 时,(b3)(c3)=9(b-3)(c-3)=9 只留下 (b,c)=(6,6).(b,c)=(6,6). 因此共有 5+3+1+1=105+3+1+1=10 个三元组。

因此,正确答案是 B

Numerically equal volume and surface area means abc=2(ab+bc+ca).abc=2(ab+bc+ca). Dividing by abcabc gives 1=2/a+2/b+2/c6/a,1=2/a+2/b+2/c\le6/a, so a6.a\le6. The cases a=1a=1 and a=2a=2 give no positive solutions. For a2,a\ne2, set u=(a2)b2a,v=(a2)c2a. \begin{gathered} u=(a-2)b-2a,\\ v=(a-2)c-2a. \end{gathered} The equation then factors as uv=4a2.uv=4a^2.

For a=3,a=3, (b6)(c6)=36(b-6)(c-6)=36 gives (b,c)=(7,42),(b,c)=(7,42), (8,24),(8,24), (9,18),(9,18), (10,15),(10,15), (12,12).(12,12). For a=4,a=4, (b4)(c4)=16(b-4)(c-4)=16 gives (5,20),(6,12),(8,8).(5,20),(6,12),(8,8). For a=5,a=5, the congruence of the two factors modulo 33 leaves only the valid pair (b,c)=(5,10),(b,c)=(5,10), and for a=6,a=6, (b3)(c3)=9(b-3)(c-3)=9 leaves only (b,c)=(6,6).(b,c)=(6,6). Thus there are 5+3+1+1=105+3+1+1=10 triples.

Thus, the correct answer is B.

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