find the sum of the geometric series

Question

3-6+12-.... to 7 terms

PLEASE HELP ME!

Answer 1

Geometric series is 3-6+12-.... to 7 terms

First term,a=3

Common ration,r=-6/3=-2

Number of terms,n=7

Recall the formulae for sum of the n terms in Geometric series

Sn=a*(1-r^n)/1-r)       (r<1)

Substitute 7 for n,3 for a and -2 for r

S7=3*(1-(-2)^7)/1-(-2))  

  =3(1-(-128))/3

 =129

 

Sum of the 7 term is 129

 

I hope that helps u