If [math]x-y=\frac{\pi}{4}[/math], then how do you prove that [math](1+\tan(x))(1+\tan(y))=2\tan(x)[/math]?
If X, Y, Z are the angles of a triangle, how do I prove that tan(X/2) tan(Y/2) + tan(X/2) tan(Z/2) + tan(Z/2) tan(Y/2) = 1?