If Sn = 1 + 6 + 11 + 16 + 21 + 26...... where an = (5n − 4), what would be the mathematical expression represented by Sk + ak + 1?
Sk = k/2 (2+5(k-1))
ak = 5k-4
So, Sk+ak+1 = 1/2 (5k^2+7k-6)
Now, if you meant a(k+1), then
Sk + a(k+1) = S(k+1)