(1+cos2a ) / (ctg a/2 - tg a/2)

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(1+cos2a)/(ctga/2-tga/2)(1+cos2a)/(ctga/2-tga/2)(1+cos2a)/(ctga/2-tga/2)(1+cos2a)/(ctga/2-tga/2)=2co

(1+cos2a ) / (ctg a/2 - tg a/2)
(1+cos2a ) / (ctg a/2 - tg a/2)

(1+cos2a ) / (ctg a/2 - tg a/2)
(1+cos2a ) / (ctg a/2 - tg a/2)
=2cos^2a/((1+cosa)/sina-(1-cosa)/sina)
=cosasina
=sin2a/2