cos2(π/4-a)=cos(π/2-2a)=sin2a
cos2（x+π/8）+sin2（x+π/8）
=cos(2x+π/4)+sin(2x+π/4)
=√2*[√2/2sin(2x+π/4)+√2/2cos(2x+π/4)]
=√2*[sin(2x+π/4)cosπ/4+cos(2x+π/4)sinπ/4]
=√2*sin(2x+π/4+π/4)
=√2*sin(2x+π/2)
=√2*cos2x