sin^4π/8+cos^4π/8的导数=0
sin^4π/8+cos^4π/8=(sin^2π/8+cos^2π/8)^2-2sin^2π/8*cos^2π/8
=1-4sin^2π/8*cos^2π/8/2
=1-(2sinπ/8*cosπ/8)^2/2
=1-(sinπ/4)^2/2
=1-1/4
=3/4