1、矢量场{y, -x}图像如下：VectorPlot[{y, -x}, {x, -3, 3}, {y, -3, 3}, Axes -> False, Frame -> False]
2、加上10条流线图，就可以看出这个矢量场的走势是一组同心圆：VectorPlot[{y, -x}, {x, -3, 3}, {y, -3, 3}, Axes -> False,Frame -> False, StreamPoints -> 10, StreamStyle -> Green]
3、矢量场{y^2 + x, y^2 - x}的图像比较混乱，所以画上36条流线。
4、{y^2 + x, x^2 + y}：VectorPlot[{y^2 + x, x^2 + y}, {x, -3, 3}, {y, -3, 3}, Axes -> False,Frame -> False, StreamPoints -> 36, StreamStyle -> Green]
5、扩大画图范围：VectorPlot[{y^2 + x, x^2 + y}, {x, -10, 10}, {y, -10, 10},Axes -> False, Frame -> False, StreamPoints -> 36,StreamColorFunction -> Hue]
6、彩色流线：VectorPlot[{y^2 + x, x^2 + y}, {x, -10, 10}, {y, -10, 10},Axes -> False, Frame -> False, StreamPoints -> 36,StreamColorFunction -> Hue]
