Tolong Bantu jawab ya

1. Dik : r = -2/5
S~ = 15
Dit : a?
Jawab : S~ = a / (1-r)
15 = a / (1-(-2/5))
15 = a / (1 + 2/5)
15 = a / ( 7/5)
15 × 5/7 = a
a = 75/7

2. Dik : a = 9/10
r = u2/u1
r = 9/100/9/10
r = 9/100 × 10/9
r = 1/10
Dit : S~?
Jawab : S~ = a / (1-r)
= 9/10 / (1-1/10)
= 9/10 / (9/10)
= 9/10 × 10/9
= 1

3.