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# help :(

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Let $$|r| < 1$$ ,

$$S = \sum_{k=0}^{\infty} r^k,$$
and
$$T = \sum_{k=0}^{\infty} k r^k.$$
Our approach is to write T as a geometric series in terms of S and r.

Give a closed form expression for T in terms of r.

May 29, 2020