cos2x-sin2x
=√2*(cos2x*√2/2 - sin2x*√2/2)
=√2*[cos2x*cos(π/4) - sin2x*sin(π/4)]
=√2*cos(2x+ π/4)我做出来答案是-√2*sin(2x- π/4)对吗？正确！cos2x-sin2x=√2*(cos2x*√2/2 - sin2x*√2/2)=√2*[cos2x*sin(π/4) - sin2x*cos(π/4)]=-√2*[sin2x*cos(π/4) - cos2x*sin(π/4)]=-√2*sin(2x- π/4)如果tanA=-1,A=（）A=-π/4 + kπ，k属于Z