tentukan banyaknya suku untuk setiap barisan geometri berikut a.5,10,20, ... , 1.280 b.-128,64,-32, ... , 1/16

a) a = 5
r = 10/5 = 2

Un = ar^(n-1)
1280 = 5 × 2^n /2
1280 × 2/5 = 2^n
512 = 2^n
2^9 = 2^n

Banyaknya suku ada 9

b) a = -128
r = 64/-128 = -1/2

Un = ar^n × r^-1
1/16 = -128 × (-1/2)^n × (-1/2)^-1
1/16×-128 = (-1/2)^n × -2
1/-2048×-2 = (-1/2)^n
1/4096 = (-1/2)^n
(-1/2)^12 = (-1/2)^n

Banyaknya suku ada 12