3 suku berurutan aritmatika (5k-7),(3k+1),(6k-1) jika nilai suku ke n adalah 59, maka jumlah n suku pertama adalah?

Deret Aritmetika
u1,u2,u3 --> sifat   2(u2) = (u1) + (u3)

2(3k+1) = (5k -7) + (6k -1)
6k + 2 = 5x -7 + 6k - 1
6k + 2 = 11 k - 8
6k  - 11 k = -8 - 2
-5 k = - 10
k = 2

a=  5k- 7 = 5(2) - 7
a = 10-7
a = 3
b = (6k -1) - (3k +1)  dgn k = 3
b = 3x - 2
b = 3(2) - 2
b = 4

Un = a +(n-1) b
59 = 3 + (n-1)4
59 = 3 + 4n - 4
59 = 4n -1
4n = 59+1 = 60
n = 60/4
n  = 15

Sn = n/2 (a + Un)
s15 = 15/2 (3 + 59)
s15 = 15/2 (62)
s15 = 15 x 31 = 465