Question

On sum of numbers in GP..
Find 81-27+9+...-1/27.

Answer 1

In the Given G.P
a=81 r=-1/3
So $81*(-1/3)^{n-1}=-1/27$
$(-1/3)^{n-1}=-1/27*81$
[tex] (-1/3)^{n-1}=-(1/3)^7 [/tex]
So,n-1=7 ,n=8
So by $S_{n}=a(1-r^n)/1-r$
We get $S_{8}=81(1-(-1/3)^8)/1-(-1/3)$
$S_{8}=1640/27$

Answer 2

Answer:

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