pake caranya yah kak

diket :

U1 = a = 3
U2 = 9
U3 = 18
U4 = 30
un = ....?
u15 =.....?

rumus = an² + bn² + c

2a = (18-9)-(9-3)
2a = 9 - 6
2a = 3
a = 1.5

3a + b= (9-3)
3a + b = 6
3(1.5) + b = 6
4.5 + b = 6
b = 6 - 4.5
b = 1.5

a + b + c = U1
a + b + c = 3
1.5 + 1.5 + c = 3
3 + c = 3
c = 3-3
c = 0

an² + bn + c

1.5 (15²) + 1.5(15) + 0
1.5(225) + 22.5
337.5 + 22.5
= 360


jadi U 15 = 360


semoga bermanfaat