Question

If the mth term of an A.P. is (2m +1), then the sum of its first three terms is​

Answer 1

m th term = 2m + 1

=> a + (m - 1) d = 2m + 1

=> a + (m - 1) d = 3 + (m - 1)2

comparing LHS and RHS

a = 3 and d = 2

therefore S3

= 3/2 { 2a + (3-1)d }

= 3/2 ( 2 x 3 + 2 x 2 )

= 3/2 x 10

= 15