Find the sum of first 20 terms of an A.P., in which 3rd term is 7 and 7th term is two more than thrice of its 3rd term.

Explanation

Let `a` be the first term and `d` be the common difference of the given `A.P`


Given,

`a_3​=7` and `a_7​=3a_3​+2`


`a+2d=7` and `a+6d=3(a+2d)+2`

`a+2d=7` and `a+6d=3a+6d+2`

`a+2d=7` and `a=3a+2`

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