Find the middle term of the A.P.. (CBSE 2015)

The sum of the first n terms of an A.P. is 3n² + 6n. Find the nth term of this A.P. (CBSE 2014)

find the middle term of the A.P. 6, 13, 20, …, 216. (CBSE 2015)

The 4^th term of an A. P. is zero. Prove that the 25^th term of the A.P. is three times its 11^th term. (CBSE 2016)

How many terms of the A.P. 18, 16, 14, … be taken so that their sum is zero? (CBSE 2016)