Find the Sn for the geometric series:

Question

a1= 243, r= -2/3, n= 5

PLEASE HELP!

Answer 1

 

First term,a1=243

Common ration,r=-2/3

Number of terms,n=5

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

Sn=a1*(1-r^n)/1-r)       (r<1)

Substitute 5 for n,243 for a and -2/3 for r

S5=243*(1-(-2/3)^5)/1-(-2/3))  

  =243(1-(-32/243))/(1+(2/3))

  =243(275/243)/(5/3)

  =275*(3/5)

 =165

 

Sum of the 5 terms is 165