Question

Suku pertama dari suatu deret geometri adl 128 dan suku keempat 16. jumlah lima suku pertama adl....

Answer 1

A= 128

U4= ar^3
16= 128 r^3
1/8 = r^3
r= 1/2

U5 = ar^4
U5 = 128 . (1/2)^4
U5 = 8

Answer 2

A = 128
u4 = ar^3 = 16
 128 . r^3 = 16
r^3 = 16/128 = 1/8
r = 1/2

S5 = a (1-r^5) / (1-r)
    = 128 (32/32 - 1/32)  / (1-1/2)
    = 128 ( 31/32) / (1/2)
    = 256 (31/32)
    = 248