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Question:
cos2a/1-sin2a=tan(45°+ a)
Answer:

Given 

      cos2a/1-sin2a

=  (cos2 a - sin2 a)/(sin2 a  +cos2 a - 2*sina*cosa)  (since cos2a =cos2 a - sin2 a and sin2 a  +cos2 a = 1)

= (cosa - sina)*(cosa + sina)/(cosa - sina)2

= (cosa + sina)/(cosa - sina)

 ={cosa*(1+ sina/cosa)}/{cosa*(1- sina/cosa)}

 = (1+ tan a)*(1 - tan a)

= (1+ tan a)*(1 - 1*tan a)

= (tan45° + tan a)*(1- tan 45° *tan a)   (since tan 45° = 1)

= tan(45° + a)

So cos2a/(1-sin2a) = tan(45° + a)

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