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# Find the sum to indicated number of terms in each of the geometric progressions in 1 minus a a square minus a cube n terms

9.  Find the sum to indicated number of terms in each of the geometric progressions in

$-a , a^2 , - a ^3 , ... n terms ( if a \neq -1)$

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The sum to the indicated number of terms in each of the geometric progressions is:

$GP=1,-a , a^2 , - a ^3 , .............$

$a=1\, \, \, and\, \, \, \, r=-a$

$S_n=\frac{a(1-r^n)}{1-r}$

$S_n=\frac{1(1-(-a)^n)}{1-(-a)}$

$S_n=\frac{1(1-(-a)^n)}{1+a}$

$S_n=\frac{1-(-a)^n}{1+a}$

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