
I decided to use inductive reasoning. First, let's assume that x=x. We know this is true for x=1. Now we can assume it is true for any value of x. Since x is arbitrary, we can take x as large as we like. If we take sufficiently large values of x, the value (x+1) is essentially equivalent, so the only important x are x less than x. So we can take x=x above, and it is also true for x+1 because x+1 is just about x.
Q.E.D. 
