The first and the last terms of an AP are 7 and 49 respectively. If sum of all its terms is 420, find its common difference.

Question

Philoid · Accepted Answer

First term, a = 7

Last term, l = 49

S_n = [a + l]

420 = [7 + 49]

840 = 56n

n = 15

Now, n = + 1

15 = + 1

15d = 42 + d

14d = 42

d = 3

Hence, it’s common difference is 3