y=(1-sinx)/(1+sinx)
1-sinx=y+ysinx
1-y=(1+y)sinx
sinx=(1-y)/(1+y)
x=arcsin((1-y)/(1+y))
因为arcsinx 值域在（-π/2,π/2)
所以反函数为y=arcsin((1-x)/(1+x))+π