6-22  ʹgradientݶȡ
⣺Enterȷϡ
>> [x,y] = meshgrid(-2:.2:2, -2:.2:2);
>> z = x .* exp(-x.^2 - y.^2);
>> [px,py] = gradient(z,.2,.2);
>> contour(z)
>> hold on
>> quiver(px,py)
>>
