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Algebra Difficulty 5.3 AIME, harder Find the answer

Compute k=01002100250+2k\sum_{k=0}^{100}\left\lfloor\frac{2^{100}}{2^{50}+2^{k}}\right\rfloor. (Here, if xx is a real number, then x\lfloor x\rfloor denotes the largest integer less than or equal to xx.)

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let ak=2100250+2ka_{k}=\frac{2^{100}}{2^{50}+2^{k}}. Notice that, for k=0,1,,49k=0,1, \ldots, 49, ak+a100k=2100250+2k+2100250+2100k=2100250+2k+250+k2k+250=250a_{k}+a_{100-k}=\frac{2^{100}}{2^{50}+2^{k}}+\frac{2^{100}}{2^{50}+2^{100-k}}=\frac{2^{100}}{2^{50}+2^{k}}+\frac{2^{50+k}}{2^{k}+2^{50}}=2^{50}. It is clear that for k=0,1,,49,ak,a100kZk=0,1, \ldots, 49, a_{k}, a_{100-k} \notin \mathbb{Z}, so ak+a100k=2501\left\lfloor a_{k}\right\rfloor+\left\lfloor a_{100-k}\right\rfloor=2^{50}-1 (since the sum of floors is an integer less than ak+a100ka_{k}+a_{100-k} but greater than ak1+a100k1a_{k}-1+a_{100-k}-1). Thus, k=0100ak=50(2501)+249=10124950\sum_{k=0}^{100}\left\lfloor a_{k}\right\rfloor=50 \cdot\left(2^{50}-1\right)+2^{49}=101 \cdot 2^{49}-50.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.