prove that a triangle with the sides given by x^2 + 1, x^2 – 1, and 2x will always be a right triangle for x > 1, using pythagorean theorem

all you have to do is probe that

(x^2 + 1)^ = (2x)^2 + (x^2-1)^2
by showing that
LS = RS

(LS is "left side")

it breaks down very nicely.