cos2a/sin(a-π/4)=-√2/5
cos2a*cos(a-π/4)/sin(a-π/4)*cos(a-π/4)
=2cos2a*cos(a-π/4)/sin(2a-π/2)
=-2cos2a*cos(a-π/4)/cos2a
=-2cos(a-π/4)=-√2/5
cos(a-π/4)=√2/10
(1)cosa+sina
=√2cos(a-π/4)
=1/5
(2)sina-cosa
=√2sin(a-π/4）
因为cos(a-π/4)=√2/10
- pai