等價無窮小條件:等價無窮小替換的誤區[朗讀]

u是自變量x、y、z的函數;設f的偏導數為f1'、f2';∂u/∂x=f1'*[∂(x/y)/∂x]+f2'*[∂(y/z)/∂x]=f1'/y+f2'*0=f1'/y;∂u/∂y=f1'*[∂(x/y)/∂y]+f2'*[∂(y/z)/∂y]=-(x/y²)f1'+(f2'/z);∂u/∂z=f1'*[∂(x/y)/∂z]+f2'*[∂(y/z)/∂z]=f1'*0-(y/z²)f2'=-(y/z²)f2';