(1-sin2x)/(1+cos2x)
=(sin²x+cos²x-2sinxcosx)/(sin²x+cos²x+cos²x-sin²x)
=(sinx-cosx)²/(2cos²x)
=(1/2)[(sinx-cosx)/cosx]²
=(1/2)(tanx-1)²
=(1/2)(2-1)²
=1/2