xy+lnx+lny=1,求d^2y/dx^2
我是用F(x,y)=xy+lnx+lny-1,求得dy/dx=-(y/x)
但d^2y/dx^2求得和答案不一样
在网上查了下,d^2y/dx^2=(-y/x)'=-(y'x-y)/x^2=-(-y/x*x-y)/x^2=2y/x^2
(-y/x)'=-(y'x-y)/x^2不明白