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 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 points. What was the total number of points scored by the two teams in the first half?
答案:E
解答:
设 Raiders 四节的得分依次为 (递增等比数列,),Wildcats 的得分依次为 (递增等差数列),两队第一节都得
将 写成最简分数,其中 因为 是整数,所以 令 Raiders 的总分为 由于 可知 数对 已使括号内的和达到 所以互质数对 只可能是
相应的 分别为 对于 和 上界迫使 但由 得到的 不是整数。对于 会得到 这在模 意义下不可能。
当 时, 且 所以 因而 再由 得 此时 Raiders 的得分是 ,Wildcats 的得分是 Raiders 以 比 获胜。
上半场总分为
因此,正确答案是 E。
Let the Raiders score (increasing geometric, ) and the Wildcats (increasing arithmetic), tied in the first quarter at
Write in lowest terms, with Since is an integer, put The Raiders' total is Since we have The pair already makes the parenthesized sum so the only possible coprime pairs are
The corresponding base values of are For and the bound forces but gives a nonintegral For it would give which is impossible modulo
For we have and so Thus and forces Then giving Raiders scores and Wildcats scores The Raiders win to
The first-half total is
Thus, the correct answer is E.
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