[x/√(x²+y²)]'=[x'·√(x²+y²)-x·√(x²+y²)']/√(x²+y²)²={√(x²+y²)-[x/2√(x²+y²)]·(2x+2y·y'}/(x²+y²)=[x²+y²-x·(x+y·y')]/(x²+y²)^(3/2)=(y²-xy·y')/(x²+y²)^(3/2)
