∵z=y/√(x²+y²)
∴dz=d(y/√(x²+y²))
=[√(x²+y²)dy-yd(√(x²+y²))]/[√(x²+y²)]²
=[√(x²+y²)dy-y(xdx+ydy)/√(x²+y²)]/[√(x²+y²)]²
=[x(xdy-ydx)]/[√(x²+y²)]³.