f(x)=(1-cosx +sinx)/(1+cosx+sinx)
=[2sin²(x/2)+2sin(x/2)cos(x/2)]/[2cos²(x/2)+2sin(x/2)cos(x/2)]
=2sin(x/2)[sin(x/2)+cos(x/2)]/2cos(x/2)[cos(x/2)+sin(x/2)]
=sin(x/2)/cos(x/2)
=tan(x/2)