1,4,7,10,is an A.P. whose first term is 1 and the common difference is equal to `4-1=3`.Find the sum of 20 terms`

Explanation

Given AP is `1,4,7,10,.....`

In given AP first term `a=1`, common difference `d=3`

Since, `S _n`​ = `n/2`​ `[2a+(n−1)d] .......` sum of n terms

Put `n=20` for sum of first `20` terms

`S_20`​ =` 20/2` `[2(1)+(20−1)3]`

`S_20``=10[2+57]`

`S_20` `=590`