2024 AMC 10A 第 7 题

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7.

三个整数的乘积为 6060。这三个整数的正的和的最小可能值是多少?

The product of three integers is 60.60. What is the least possible positive sum of the three integers?

22

33

55

66

1313

答案:B
知识点:因数最优化分类讨论
难度评级:1200
解答:

正乘积来自三个正整数,或一个正整数与两个负整数。三个正整数的和至少为 33。在第二种情形中,把三个数写成 x,y,z-x,-y,z,其中 x,y,zx,y,z 均为正数且 xyz=60xyz=60。若三数之和为正,则 z>x+y2xyz \gt x+y \ge 2\sqrt{xy},所以 60/(xy)>2xy60/(xy) \gt 2\sqrt{xy},从而 xy<10xy \lt 10。检查 6060 的因数中满足 xyxy 的因数对,1,2,3,4,5,61,2,3,4,5,6 的最小正值在 60/(xy)xy60/(xy)-x-y 时取得,等于 33。因此 (1)(6)(10)=60(-1)(-6)(10)=60,最小正和为 1016=310-1-6=3,正确答案是 B

A positive product comes from three positive integers or one positive and two negative integers. Three positive integers have sum at least 3.3. In the second case write the numbers as x,y,z,-x,-y,z, where x,y,zx,y,z are positive and xyz=60.xyz=60. A positive sum requires z>x+y2xy,z \gt x+y \ge 2\sqrt{xy}, so 60/(xy)>2xy60/(xy) \gt 2\sqrt{xy} and hence xy<10.xy \lt 10. The possible products xyxy that divide 6060 are 1,2,3,4,5,6.1,2,3,4,5,6. Checking their factor pairs, the smallest positive value of 60/(xy)xy60/(xy)-x-y is 1016=3.10-1-6=3. Thus (1)(6)(10)=60,(-1)(-6)(10)=60, and no positive sum below 33 is possible. Therefore, the answer is B.

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