Problem 741

Sir SagarMay 22, 2015May 22, 20150

Find the sum of the first 20 terms of the sequence: -1, 1, -1, 1, -1, …

Solution:-

Find a_n by using the explicit formula for geometric sequences: a_n= a₁r^n-1 . Then use the formula for the sum of a finite geometric series:

S_n = $\frac{a_{1}-a_{1}r^{n}}{1-r}$

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