1-tanx=(cosx-sinx)/cosx
1+sin2x+cos2x
=(1+cos2x)+sin2x
=2cos^2x+2sinxcosx
=2cosx(sinx+cosx)
所以相乘等于：
=[(cosx-sinx)/cosx]×[2cosx(sinx+cosx)]
=2[cos^2x-sin^2x]
=2cos2x