Question

How many terms of an AP 3‚8‚13‚18...... must be taken to get their sum as 78.¿??? CN any one plzz answer this Quickly because I am in hurry....,,

Answer 1

$a = 3 \\ d = 5 \\ S_{n} = \frac{n}{2} (2a + (n - 1)d) \\ 78 = \frac{n}{2} (2 \times 3 + (n - 1) \times 5) \\ 156 = n(6 + 5n - 5) \\ 156 = 5 {n}^{2} + n \\ 5 {n}^{2} + n - 156 = 0$