jika -2+4-8+16+.....+Un=-170, banyak suku deret tersebut

suku pertama a = -2, rasio r = 4/-2 =-8/4 = -2
Sn = a ( r^n - 1 ) / (r - 1)
170 = -2 ( -2^n - 1 ) / (-2-1)
170 = ( -2^(n +1) + 2 ) / -3
-2^(n+1) + 2 = -510
-2^(n+1) = -512
(-2)^(n+1) = (-2)^9
n+1 = 9, n =8
Jadi banyak suku = 8