4 find the sum of the first eight terms in the geometric ser

4. find the sum of the first eight terms in the geometric series -4 + 8 - 16 +...

Solution

S = -4 +8 -16 ...

G.P. common ratio , r = 8/-4 = -2

Formula for sum of G.P. = a(1 -r^n)/(1-r)

n= 8 ; a= -4

Sum = -4( 1- (-2)^8)/(1- (-2) )

=-4[ 1-256]/3

= -4*255/3

= -4*85

= -340