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

Given that, a = 9, d = 8 and S_n = 636

Now

We know:

(4N +53)(N – 12)

Now, n = - 53/4 and n = 12

Taking the integral value and rejecting the fractional value, we have n = 12