please jawab big point

3a)
r = u2/u1 = 2/1 = 2
a = 1

U10 = ar^9 = 1.(2)^9 = 512
S10 = 1(2^10-1) / 2-1 = 1023

3b)
r = u2/u1 = 18/54 = 1/3
a = 54

U8 = ar^7 = 54(1/3)^7 = 54/2187
S8 = 54 (1-(1/3)^8) / 1-1/8
S8 = 54 (1-1/6561) / 7/8
S8 = 54 (6560/6561) / 7/8
S8 = 2833920/45927

3c)
r = u2/u1 = -6/2 = -3
a = 2

U7 = ar^6 = 2(-3)^6 = 1458
S7 = 2 (1-(-3)^7) / 1-(-3)
S7 = 2 (1+2187) / 4
S7 = 1094

semoga membantu.
ㅡ :)