Let a1, a2, a3, ... be an arithmetic series. Let Sn denote the sum of the first n terms. If S1 = 5 and S2 = 15, then find S15.
S1 = 5
S2 = 15
Then
a1 = 5 and a2 = 10 d = 5
So
a15 = a1 + (15 - 1) (d) = 5 + (15 - 1) (5) = 5 + 14 * 5 = 75
S15 =[ a1 + a15 ] ( 15 / 2) = [ 5 + 75 ] (15 / 2) = 600
+291 
