tany=y/xsec²ydy=(xdy-ydx)/x²而sec²y=1+tan²y=1+(y/x)²=(x²+y²)/x²所以有(x²+y²)/x²*dy=(xdy-ydx)/x²(x²+y²-x)dy=-ydxdy/dx=-y/(x²+y²-x)
