Difference between revisions of "G285 2021 Fall Problem Set Problem 8"
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Find <cmath>\sum_{a=0}^{\infty} \sum_{b=0}^{\infty} \sum_{c=0}^{\infty} \frac{a+2b+3c}{4^{(a+b+c)}}</cmath> | Find <cmath>\sum_{a=0}^{\infty} \sum_{b=0}^{\infty} \sum_{c=0}^{\infty} \frac{a+2b+3c}{4^{(a+b+c)}}</cmath> | ||
− | < | + | <math>\textbf{(A)}\ \frac{16}{27} \qquad \textbf{(B)}\ \frac{32}{27} \qquad \textbf{(C)}\ \frac{64}{27} \qquad \textbf{(D)}\ \frac{128}{27} \qquad \textbf{(E)}\ \frac{256}{27}</math> |
==Solution== | ==Solution== |
Revision as of 22:37, 11 July 2021
Problem
Find
Solution
We begin with a simpler problem . Now, suppose and are constant. We have a converging geometric series for with a sum of . Now, make everchanging. We have , so the entire sum must be .
Now, coming back to the original problem, we split the single sum into : Split into single variables to get Now, generalize to obtain . Using the geometric series formula we have Now, we can plug this in for all to get