This is an expansion of the pythagorean theorem using sin and cos^-1, because sin(cos^-1(x))=cos(sin^-1(x))=(1-x^2)^1/2. You can derive this by taking the sin of the relationship between arccos, arcsin and pi. This definition can also be expanded to the difference between any two numbers, functions, etc inside a square root. Those relationships are graphed here. Note that this means related equations overlap, also the domains may be undefined for negative x values in some cases.
