So yeah, basically I got my GCSE Maths result back, and I only dropped one mark, but I'm not sure how or where I lost it. I'll post my method, and then you guys can tell me what I did wrong.

Question: Prove that the sum of two consecutive odd integers is always even.

My method;

Let two consecutive odd integers be

and

.

Hence,

Since twice anything is always even;

, which is true. Therefore, the sum of two consecutive odd integers is always even. Any idea where I dropped the mark? I only got two of a possible three.