x^2+xy+y^2=4
全微分,所以
2xdx+xdy+ydx+2ydy=0
(2x+y)dx+(x+2y)dy=0
dy/dx=-(2x+y)/(x+2y)
将(2,0)代入可得,dy/dx=-4/2=-2
切线方程就是y=-2x+4