f（X）=cosx(sinx-cosx)+1
=sinxcosx-(cosx)^2+1
=1/2sin2x-(sinx)^2
=1/2sin2x-(1-cos2x)/2
=sin2x/2+cos2x/2-1/2
=√2/2(sin2x*√2/2+cos2x/2*√2/2)-1/2
=√2/2sin(2x+π/4)-1/2
T=2π/2=π
-1