sum(8..15) = sum(1..15) - sum(1..7)
so, since a=89 and d=14, you have
15/2 (2*89 + 14*14) - 7/2 (2*89 + 6*14) = 1888
Given the sequence: 89, 103, 117, 131, ……, find
the sum of teams from the 8th to the 15th inclusive
1 answer