cos2x/cos(π/4+x)=sin(2x+π/2)/cos(π/4+x)
=2sin(π/4+x)cos(π/4+x)/cos(π/4+x)
=2sin(π/4+x)
=2cos(π/4-x)
因为0 cos2x/cos(π/4+x)=sin(2x+π/2)/cos(π/4+x)
=2sin(π/4+x)cos(π/4+x)/cos(π/4+x)
=2sin(π/4+x)
=2cos(π/4-x)
因为0