2000 AIME II Problem 7

Attempt Problem 7 of the 2000 AIME II below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2000 AIME II solutions, or check the answer key.

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7.

Given that 12!17!+13!16!+14!15!+15!14!+16!13!+17!12!+18!11!+19!10!=N1!18!, \begin{aligned} &\frac{1}{2!17!} + \frac{1}{3!16!} + \frac{1}{4!15!} \\ &\quad {}+ \frac{1}{5!14!} + \frac{1}{6!13!} + \frac{1}{7!12!} \\ &\quad {}+ \frac{1}{8!11!} + \frac{1}{9!10!} = \frac{N}{1!18!}, \end{aligned} find the greatest integer that is less than N100.\frac{N}{100}.

Answer: 137
Concepts:binomial theoremfactorialsymmetry (algebra)
Difficulty rating: 2360
Solution:

Multiply both sides by 19!.19!. Each term on the left becomes 19!k!(19k)!=(19k)\frac{19!}{k!\,(19 - k)!} = \binom{19}{k} for k=2,,9,k = 2, \ldots, 9, while the right side becomes 19N.19N.

Since (19k)=(1919k),\binom{19}{k} = \binom{19}{19 - k}, the first half of the binomial row sums to k=09(19k)=2192=218,\sum_{k=0}^{9} \binom{19}{k} = \frac{2^{19}}{2} = 2^{18}, so k=29(19k)=218(190)(191)=26214420=262124. \begin{aligned} \sum_{k=2}^{9}\binom{19}{k} &= 2^{18} - \binom{19}{0} \\ &\quad {}- \binom{19}{1} \\ &= 262144 - 20 \\ &= 262124. \end{aligned}

Hence N=26212419=13796,N = \frac{262124}{19} = 13796, so N100=137.96,\frac{N}{100} = 137.96, and the greatest integer less than this is 137.137.

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