Question

Sebenernya udah tau jawabannya,tapi gak tau cara dapetnya :v

2,3,4,5...n
Sn = 20
berapakah n?

Answer 1

A = 2
b = 3-2 = 4-3 = 5-4 = 1

Sn = 20
n/2 (2a+(n-1).b) = 20
n/2 (2.2+(n-1).1) = 20
n/2 (4+n-1) = 20
n(4+n-1) = 20.2
n(3+n) = 40
3n+n²-40 = 0
n²+3n-40 = 0
(n-5)(n+8) = 0
n-5 = 0
n = 0+5
n = 5
atau
n+8 = 0
n = 0-8
n = -8