The LHS part of the equation is:
tan[π4+12cos−1ab]+tan[π4−12cos−1ab]letcos−1ab=x⇒cosx=abtan[π4+x2]+tan[π4−x2]
=tanπ4+tanx21−tanπ4tanx2+tanπ4−tanx21+tanπ4.tanx2=1+tanx21−tanx2+1−tanx21+tanx2
=(1+tanx2)2+(1−tanx2)2(1−tanx2)(1+tanx2)=2(1+tan2x2)1−tan2x2
=2cosx[since cos 2x=1−tan2x21+tan2x2]
=2ba