Looking at the "

Square of an Odd" puzzle that asks to prove that the square of an odd number is always 1 more than a multiple of 8, a professor gave this four parts proof: "All odd numbers are of the form 8K+1, 8K+3, 8K+5 or 8K+7. Squaring these numbers produces 8M+1, 8M+9, 8M+25 or 8M+49, which are all of the form 8N+1. QED"

Another professor came by, and gave a single line proof. Can you manage it?

*Note: no one who answered the original problem produced either the four parts solution, or the single line one.*

7 years after it was posted:

**x^2-1=(x-1)*(x+1) ... one line to look at**

if x is odd then we have a product of two succesive even numbers, and one of them must be divisible by 4.

qed