/ *画圆的半径为9，8和7所示的颜色（0,0）

代码说明：

/* Draw circles of radius 9, 8 and 7 with the colours shown around (0,0) Draw the x and y axis Draw the diagonal line Draw the 12 by 6 quad around (0,0) When the left mouse button is pressed the diagonal line moves as shown. When the right button is pressed the diagonal line is reset to its original position The right button also toggles the direction from + to - x Pressing Esc exits the program */-/* Draw circles of radius 9, 8 and 7 with the colours shown around (0,0) Draw the x and y axis Draw the diagonal line Draw the 12 by 6 quad around (0,0) When the left mouse button is pressed the diagonal line moves as shown. When the right button is pressed the diagonal line is reset to its original position The right button also toggles the direction from+ to- x Pressing Esc exits the program */