Let a1, a2, a3, … be an arithmetic sequence. Let Sn denote the sum of the first n terms. If S20=15 and S10=0, then find S70.
0 + 55d = 1/5
55d = 1/5
d = 1 / [ 5 * 55] = 1 / 275
70th term is 0 + 60d = 60 / 275 =12 / 55
S70 = [ 12/55 + 0 ] [70 -10 + 1 ] / 2 =
[12/55] * 61 / 2 =
366 / 55
+1266 
