Question

Diketahui barisan geometri dengan suku ketiga U3 = 8 dan suku kelima U5 = 32. jumlah sembilan suku pertama barisan tersebut adalah...

Answer 1

U5 = ar^4 = 32
= ar² x r² = 32
= 8 x r² = 32
r² = 32 : 8
r² = 4
r = 2

U3 = ar²
8 = a x 2²
8 = a x 4
8 : 4 = a
2 = a

S9 = a(r^n - 1) per r - 1
= 2 (2^9 - 1) per 2 - 1
= 2 (512 - 1) per 1
= 2 x 511
= 1022