Question 1035623
7 + 9 + 11 + ... + (2k + 5) + (2(k + 1) + 5) = k(k + 6) +  2(k + 1) + 5
= {{{k^2 + 6k + 2k + 7 = k^2 +8k + 7 = (k+1)(k+7) = (k+1)((k+1)+ 6)}}}