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Algebra Difficulty 5.2 AIME, harder Find the answer

Compute the sum of all positive real numbers x5x \leq 5 satisfying x=x2+xxx+xx=\frac{\left\lceil x^{2}\right\rceil+\lceil x\rceil \cdot\lfloor x\rfloor}{\lceil x\rceil+\lfloor x\rfloor}.

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

Solution

Note that all integer xx work. If xx is not an integer then suppose n<x<n+1n<x<n+1. Then x=n+k2n+1x=n+\frac{k}{2n+1}, where nn is an integer and 1k2n1 \leq k \leq 2n is also an integer, since the denominator of the fraction on the right hand side is 2n+12n+1. We now show that all xx of this form work. Note that x2=n2+2nk2n+1+(k2n+1)2=n2+kk2n+1+(k2n+1)2x^{2}=n^{2}+\frac{2nk}{2n+1}+\left(\frac{k}{2n+1}\right)^{2}=n^{2}+k-\frac{k}{2n+1}+\left(\frac{k}{2n+1}\right)^{2}. For k2n+1\frac{k}{2n+1} between 0 and 1, k2n+1+(k2n+1)2-\frac{k}{2n+1}+\left(\frac{k}{2n+1}\right)^{2} is between 14-\frac{1}{4} and 0, so we have n2+k1<x2n2+kn^{2}+k-1<x^{2} \leq n^{2}+k, and x2=n2+k\left\lceil x^{2}\right\rceil=n^{2}+k. Then, x2+xxx+x=n2+k+n(n+1)2n+1=n+k2n+1=x\frac{\left\lceil x^{2}\right\rceil+\lceil x\rceil \cdot\lfloor x\rfloor}{\lceil x\rceil+\lfloor x\rfloor}=\frac{n^{2}+k+n \cdot(n+1)}{2n+1}=n+\frac{k}{2n+1}=x so all xx of this form work. Now, note that the 2n2n solutions in the interval (n,n+1)(n, n+1), together with the solution n+1n+1, form an arithmetic progression with 2n+12n+1 terms and average value n+n+12n+1n+\frac{n+1}{2n+1}. Thus, the sum of the solutions in the interval (n,n+1](n, n+1] is 2n2+2n+1=n2+(n+1)22n^{2}+2n+1=n^{2}+(n+1)^{2}. Summing this for nn from 0 to 4, we get that the answer is 02+2(12+22+32+42)+52=850^{2}+2\left(1^{2}+2^{2}+3^{2}+4^{2}\right)+5^{2}=85.

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