cos^4*x-2sinxcosx-sin^4*x题目错了,应该是:
cos^4x-2sinxcosx-sin^4x=(cos^2x+sin^2x)(cos^2x-sin^2x)-sin2x
=cos2x-sin2x=√2(cos45*cos2x-sin45*sin2x)
=√2cos(45+2x)
因x属于[0,90度],故45+2x属于[45,225]
max=√2cos45=1min=√2cos180=-√2