tentukan banyaknya suku untuk setiap barisan geometri berikut a. 5,10,20,....,1280 b. -128,64,-32,....,1/16

Un = ar^n-1
a. a=5, r =2
1280=5*2^n-1
256 = 2^n-1
2^16 = 2^n-1
8 = n-1
n = 9

b.a=-128, r=-½
1/16 = -128*(-½^n-1)
(1/16)/-128 = -½^n-1
-8 = -½^n-1
½^n-1 = 8
2^n-1= 2³
n-1 = 3
n = 4
n = -2