对x的偏导数是6x+1 ---> f(x,y) = 3x^2+x+C(y)f(x,2x)=x^2+3x -----> 3x^2+x+C(2x) = x^2+3x ------> C(2x) = -2x^2 + 2x------> C(x) = x-1/2x^2 -----> C(y) = y - 1/2y^2f(x,y) = f(x,y) = 3x^2+x+y - 1/2y^2f(x,...C(2x) = -2x^2 + 2x这部是为什么？ C(x) = x-1/2x^2你可以令u=2x---> x=u/2 C(2x) = -2x^2 + 2xC(u)=-2(u/2)^2+2(u/2)=u-1/2u^2------>C(x) = x-1/2x^2