[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) =(y。
