8. Sequences and Series
easy

સમગુણોત્તર શ્રેણીમાં નિર્દેશિત પદોનો સરવાળો શોધો : ${1, - a,{a^2}, - {a^3}, \ldots }$ પ્રથમ $n$ પદ

A

$\frac{\left[1-(-a)^{n}\right]}{1+a}$

B

$\frac{\left[1-(-a)^{n}\right]}{1+a}$

C

$\frac{\left[1-(-a)^{n}\right]}{1+a}$

D

$\frac{\left[1-(-a)^{n}\right]}{1+a}$

Solution

The given $G.P.$ is $1,-a, a^{2},-a^{3} \ldots \ldots$

Here, first term $=a_{1}=1$

Common ratio $=r=-a$

$S_{n}=\frac{a_{1}\left(1-r^{n}\right)}{1-r}$

$\therefore S_{n}=\frac{1\left[1-(-a)^{n}\right]}{1-(-a)}=\frac{\left[1-(-a)^{n}\right]}{1+a}$

Standard 11
Mathematics

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