Question

Find the sum of first 20 terms of the following A.P. : 1, 4, 7, 10, ........

Answer 1

Let a be the first term and d be the common difference of the given A.P. Then, we have a = 1 and d = 3.

We have to find the sum of 20 terms of the given A.P.

Putting a = 1, d = 3, n = 20 in

`S_n = \frac{n}{ 2 } [2a + (n - 1) d]`

∴ `S_20 = \frac {20}{2} [2 × 1 + (20 - 1) × 3]`

= 10 (2 + 57)

= 10 × 59

= 590