原式=(cos^2x+sin^2x)(cos^2x-sin^2x)-2sinxcosx
=cos^2x-sin^2x-2sinxcosx
=cos2x-sin2x
=√2[(√2/2)cos2x-(√2/2)sin2x]
=√2cos(π/4+2x)
周期2π/2=π
当π/4+2x=π+2kπ时,即x=3π/8+kπ时,f(x)最小=-√2