x∈[π,3π/2]时. x-π∈[0,π/2]
sin(x-π)=-sinx=-a
x-π=arcsin(-a)=-arcsina
x=π-arcsina
x∈[3π/2,2π]时. x-2π∈[-π/2, 0]
sin(x-2π)=sinx=a
x-2π=arcsina
x=2π+arcsina