cos(x-y)=cosxcosy-sinxsiny sin(x+y)=sinxcosy+cosxsiny
上式左边=(cosx)^2cosy-sinxcosxsiny+(sinx)^2cosy+sinxcosxsiny
=cosy=m
所以,siny=+-（1-m^2）^(1/2) 因为y为第三象限角,所以siny为负
所以,siny=-(1-m^2)^(1/2)