Question

Mohon jawabannya pak guru bu guru dan teman sekalian.
Terimakasih.

Answer 1

Verified answer

Jawaban C. 81
Cara bisa dilihat di gambar ya

Answer 2

Verified answer

Barisan Geometri 
jumlah tak hingga  S~ = a/(1 - r)
U1 + U2 = 45 --> a  + ar = 45
U3 + U4 = 20 --> ar² + ar³ = 20 --> r²(a + ar) = 20
r² (45) = 20
r² = 20/45 = 4/9
r = 2/3
.
a + ar = 45
a + a(2/3)  = 45...kalikan 3
3a + 2a = 135
5a =135
a = 27
.
Jumlah tak hngga
S~ = a / (1 - r)
S~ = 27/ ( 1 - 2/3) = 27/(1/3) = 27(3)
s~ = 81