Question

How many terms of the AP : 9, 17, 25, . . . must be taken to give a sum of 636?

Answer 1

The A.P. is given by  9, 17, 25, .........

First term, a = 9

Common difference, d = 17 - 9 = 25 - 17 = 8

Let n be the number of terms for which the sum is 636.

∴Sum of n terms, S_n = 636

As the number of terms can not be negative, therefore, n = 12.