f(x)=cos^4x-2sinxcosx-sin^4x
=(cos²x+sin²x)(cos²x-sin²x)-2sinxcosx
=1×cos2x-sin2x;
=√2(cos2x×(√2/2)-sin2x×(√2/2))
=√2sin(2x-π/4)
如果本题有什么不明白可以追问,