AP is 9, 17, 25,…
a = 9, d = 17 – 9 = 8
Sum of terms = Sn = 636
(n/2) [2a + (n – 1)d] = 636
(n/2) [2(9) + (n – 1)(8)] = 636
(n/2)[10 + 8n] = 636
4n2 + 5n – 636 = 0 (which is a quadratic equation)
(n- 12)(4n + 53) = 0
Either (n- 12) = 0 or (4n + 53) = 0
n = 12 or n = – 53/4
Since n can’t be negative and fraction, so
n = 12
Number of terms = 12 terms.
