In the 2md and 4th term of a G.p are 8 and 32 respectively what is the sum of the four term

Answer 1

r^2 = 32/8 = 4
r = ±2
a = ±4
S4 = a(r^4-1)/(r-1)
so the sum is either
4*15/1 = 60
or
-4*15/-3 = 20