Problem 741

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

 

Solution:-

 

Find  an  by using the explicit formula for geometric sequences: an = a1rn-1   . Then use the formula for the sum of a finite geometric series:

Sn =  \frac{a_{1}-a_{1}r^{n}}{1-r}

0.

 

 

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