Dear nishat

In Triangle ACD 

 use sine rule

a/2 sinB /sin(180-A-B)   = a/2 /sin(A-90)

sinB/sin(A+B)  = -1/cosA

 sinB cosA = -sin(A+B)

 sinBcosA  =-[sinAcosB + cosA sinB]

 tanA +2tanB =0

post seperate question in different post

     
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