If a4= 4 of an AP then find the sum of first 7 terms of the corresponding arithmetic series

Question

If im given with the 4th value and the total terms how do I solve without a1 or an and d

 

Answer 1

a₄ = 4  Let d = common difference.

 

First 7 terms:  4-3d, 4-2d, 4-d, 4, 4+d, 4+2d, 4+3d

 

Sum of first 7 terms = 4(7) = 28