Diketahui U1 = 4 dan S8 = 17 . S4. Hitunglah r nya!

u1 = a = 4
S8 = 17*S4
a(r^8-1)/(r-1) = 17*a(r^4-1)/(r-1)
r^8-1 = 17(r^4-1)
r^8-1 = 17r^4 - 17
r^8 - 17r^4 +16 = 0
(r^4 - 1)(r^4 - 16)=0
r^4 = 1 V r^4 = 16
r=+-1 V r=+-4