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20. U4=ar^3 = 2
U6=ar^5 = 1/2
Eliminasi.
r^2 X 2 = 1/2
r^2 = 1/4
r = 1/2

U4 = ar^3
2 = a (1/2)^3
2 = a 1/8
16 = a

U2 = ar
= 16 X 1/2
= 8 ( D )

21. U5=ar^4 = 48
U8=ar^7 = 384
Eliminasi
r^3 X 48 = 384
r^3 = 8
r = 2

U5 = ar^4
48 = a (2)^4
48 = a 16
3 = a

U4 = ar^3
= 3 X 2^3
= 3 X 8
= 24 ( A )