Let a1, a2, a3, … be an arithmetic sequence. Let Sn denote the sum of the first n terms. If S1=15 and S2=110, then find S15.
S1 = a1 = 1/5
S2 = 2a1 + d
1/10 = 2/5 + d
1/10 - 2/5 = d
d = -3/10
sum of the first n terms of an arithmetic sequence =
(n)*a1 + (n)(n-1)/2 *d
S15 = (15 ) (1/5) + (15)(14)/2 * (-3/10) = 3 - 63/2 = -57/2 = - 28.5
+216 
