pakai cara ya no 9 dan 10

9][
Pola :
3 , 9 , 18 , ...
6 , 9 , 12 , ...
3 , 3 , ...

2A = 3
A = 3/2

3A + B = 6
9/2 + B = 6
B = 3/2

A + B + C = 3
C = 0

Un = An² + Bn + C
Un = 3/2 (n² + n)

U15 = 3/2 (15² + 15)
U15 = 3/2 × 240
U15 = 360

10][
U4 = ar³ = 40
5r³ = 40
r³ = 8
r = 2

Sn = a(r^n - 1)/(r - 1)
S6 = 5(2^6 - 1)
S6 = 5 × 63
S6 = 315