S4=-40, q=-3 , найти s8

Question

S4=-40, q=-3 , найти s8

Answer 1

S4 = a1.(qˇ4-1)/q-1
-40=a1.(81-1)/(-3-1)
-40=a1.80/(-4)
-40=a1.(-20), a1 = 2
s8 = a1. (qˇ8-1)/(-3-1)
s8 = 2. (6561-1)/(-4) =-1/2.6560=-3280

Answer 2

S4*q^4=-40*81=-3240 .....