(2sin2a/(1+cos2a))*((cosa^2)/cos2a)=
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(2sin2a/(1+cos2a))*((cosa^2)/cos2a)=
(2sin2a/(1+cos2a))*((cosa^2)/cos2a)=
(2sin2a/(1+cos2a))*((cosa^2)/cos2a)=
cos2a=2(cosa)^2-1
所以1+cos2a=2(cosa)^2
所以原式=2(sin2a/cos2a)*(cosa)^2/[2(cosa)^2]
=tan2a
(2sin2a/(1+cos2a))*((cosa^2)/cos2a)=
=2tan2a*cosa^2/(2cosa^2)
=tan2a