If cos A= mcos B and cot(A+B)2=λtan(B−A2) and λ is
cosAcosB=m1⇒cosA+cosBcosA−cosB=m+1m−1 =2cos(A+B2)cos(B−A2)2sin(A+B2)sin(B−A2)=m+1m−1 ⇒cot(A+B2)=(m+1m−1)tan(B−A2)