Question

11. Find the sum of first 40 terms of an AP whose 2nd term is 2 and 9th term is 37.​

Answer 1

3780

Explanation:

First we need to find the common difference (d)

d = (37-2)/7 = 5 (7 jumps from term 2 to term 9)

Second we need to find the first term (a1) using the second term

a1 = 2-5 = -3

Third use the sum formula:

Sn = n/2 x (2a1 +d(n-1))

S40 = 40/2 x (2x-3 + 5(40-1)) = 3780