Question

Question #1df71

Answer 1

Given that first term of geometric series `a=-11`,
the common ratio `r=-4`,
the last or 8 th term `t_8=ar^7=180224`

Now

`S_8=a+ar+ar^2+ar^3+.......ar^7----(1)`

`rS_8=ar+ar^2+ar^3+......+ar^7+ar^8----(2)`

subtracting (2) from (1) we get

`(1-r)S_8=a-ar^8=a-rxxar^7`

`=>S_8=(a-rxxt_8)/(1-r)=(-11-(-4)xx180224)/(1-(-4))`

`=(-11+720896)/5=144177`