dy/dx=2/(2-cosy)
即：
y'=2/(2-cosy)
y''
=-(2-cosy)'/(2-cosy)^2
=(cosy)'/(2-cosy)^2
=-siny*y'/(2-cosy)^2,将y'代入得到：
d^2y/dx^2 =y''=-2siny/(2-cosy)^3.