列一个三行的表：  中间一行为0,π/2,π,3π/2,2π,这是五点法的关键.
x                       π/12      π/3      7π/12        5π/6     13π/12           
2x-π/6               0          π/2          π           3π/2        2π
y                        0           3            0             -3            0