1、f(x)=cos²x/2-sinx/2cosx/2-1/2
=1/2(cosx+1)-1/2sinx-1/2
=1/2(cosx+sinx)
=√2/2sin(x+π/4)
f(π/4) =√2/2sin(π/4+π/4) =√2/2
2、0