绘制引力线图，大家一起讨论！

wem

1. [x,y]=meshgrid(-3:.2:3, -2:.2:2);
   
2. z=x.^2.*exp(-x.^2-y.^2);
   
3. [fx,fy]=gradient(z,0.2,0.2);
   
4. contour(x,y,z,30);
   
5. hold on; quiver(x,y,fx,fy)

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