Question

Suku pertama 4 dan beda 2. jk jumlah n suku pertama adl 180 mk n=...

Answer 1

A = 4, b = 2
Sn = 180
Sn = n/2 (2a+(n-1)b)
180 = n/2 (2.4+(n-1)2)
360 = n(8+2n-2) = n(2n+6)
360 = 2n^2+6n
2n^2+6n-360 = 0
n^2+3n-180 = 0
(n+15)(n-12)=0
n = -15 (Tak mungkin n negatif)
n = 12