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.
First term = 1 = a
Second term = 14
15th term = 1 + 14 * 13 = 183
S15 = [ first term + last term ] (no.of terms / 2) =
]1 + 183 ] / (15/2) =
1380 = S15
+479 
