selesaikan barisan geometri jika diketahui suku ke 4=27 dan suku ke6=243 tentukan suku ke8

U4 = 27
27 = ar^4-1
27 = ar³ ....(1)

U6 = 243
243 = ar^6-1
243 = ar^5 ....(2)

27 = ar³
243 = ar^5
------------------ :
27/243 = r³/r^5
1/9 = r³. r^-5
1/9 = r^-2
r² = 9
r = 3

27 = ar³
27 = a(3)³
27 = 27a
a = 1

maka, U8 = ar^8-1
U8 = 1.(3)^7
U8 = 2187