已知sina=-m,则cosa=_______

已知sina=-m,则cosa=_______
已知sina=-m,则cosa=_______

已知sina=-m,则cosa=_______
cos2a=2cos^2a-1=1-2sin^2a
1-2sin^2a=1-2(-m)^2=1-2m^2
即2cos^2a-1=1-2m^2
2cos^2a=2-2m^2