How do you prove ∞∑n=1n2n=2 ?
My attempt: I have been trying to find geometric series that converge to 2 which can bind the given series on either side. But I am unable to find these. Is there a general technique to find the sum? This is a high school interview question and must be easy enough to solve in a few minutes.
Please give any hints for the first step towards a solution.
Answer
Hint: Denote your sum by S, rewrite S=2S−S and try to rewrite the sums.
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