Question

if nth term of an ap is 2n + 1 find S3​

Answer 1

your answer is here

Answer 2

1st term : 2 X 1 + 1

             = 3

2nd term : 2 X 2 +1

                = 5

So, common difference : 5 - 3

                                        = 2

Then, S3 =  n/2 [2a + (n-1) d]

               = 3/2 [2 X 3 + ( 3 - 1 ) 2]

               = 3/2 [ 6 + 4 ]

               =  3/2 X  10

               =  3 X 5

               = 15 (Ans)