jika barisan aritmatika 2,6,8,20... maka berapakah U20?

2,6,12,20
B1 = 4,6,8
B2 = 2,2

2a = 2
a = 1
3a+b = 4
b = 1
a+b+c = 2
1+1+c = 2
c = 0

Un = n² + n
U20 = 20² + 20 = 420