已知z=y/f(x2-y2),其中f可微,证明1/x(∂z/∂x)+1/y(∂z/∂y)=z/y2.
∂z/∂x=2xyf′(x2-y2)/f(x2-y2) ∂z/∂y=[f(x2-y2)+2y2f′(x2-y2)]/[f′(x2-y2)] 所以 1/x(∂z/∂x)+1/y(∂z/∂y)=-[2xf′(x2-y2)/f2(x2-y2)]+1/[yf(x2-y2)]+2xf′(x2-y2)/f(x2-y2) =1/[yf(x2-y2)]=1/y2(y/f(x2-y2))=z/y2
已知z=y/f(x2-y2),其中f可微,证明1/x(∂z/∂x)+1/y(∂z/∂y)=z/y2.
∂z/∂x=2xyf′(x2-y2)/f(x2-y2) ∂z/∂y=[f(x2-y2)+2y2f′(x2-y2)]/[f′(x2-y2)] 所以 1/x(∂z/∂x)+1/y(∂z/∂y)=-[2xf′(x2-y2)/f2(x2-y2)]+1/[yf(x2-y2)]+2xf′(x2-y2)/f(x2-y2) =1/[yf(x2-y2)]=1/y2(y/f(x2-y2))=z/y2