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 the second term is 10 and d = 5
15th term is 5 + 14(5) = 75
S15 = [ 15th term + first term ] * no.of terms / 2 =
[ 75 + 5 ] (15 / 2) = 80 * 15/2 = 40 * 15 = 600
+866 
