find the sum of the geometric series using a formula 1-4+16-64+256-1024

Answer 1

a = 1 , r = -4

you are showing 6 terms, so
sum(6) = a(r^6 - 1)/(r-1)
= 1((-4)^6 - 1)/(-4-1)
= 4095/-5
= -819