f(x,y)=xy^3/(x^2+y^4),df=(y^3dx+3xy^2dy)/(x^2+y^4)-xy^3(2xdx+4y^3dy)/(x^2+y^4)^2=[(y^3dx+3xy^2dy)(x^2+y^4)-(2x^2y^3dx+4xy^6dy)]/(x^2+y^4)^2=[(-x^2y^3+y^7)dx+(3x^3y^2-xy^6)dy]/(x^2+y^4)^2,在(0.0)处可微。
