Question

Find the sum of 20 terms of the 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 × 59 = 590