NOTASI SIGMA No10 pake cara ya!

Sn = n/2 (a + Un)

Sigma (4k + 5) = 774
(4.1 + 5) + (4.2 + 5) + (4.3 + 5) + ... + (4n + 5) = 774
9 + 13 + 17 + ... + (4n + 5) = 774
Sn = 774
n/2 (a + Un) = 774
n/2 (9 + (4n + 5)) = 774
n/2 (4n + 14) = 774
2n^2 + 7n - 774 = 0
(2n + 43)(n - 18) = 0
n = -43/2 atau n = 18
Ambil yang positif maka n = 18