Solution:
To Prove : 1cos(x−a)cos(x−b)=tan(x−b)−tan(x−a)sin(a−b)
R.H.S.
tan(x−b)−tan(x−a)sin(a−b)
=sin(x−b)cos(x−a)−sin(x−a)cos(x−a)sin(a−b)
=sin(x−b)cos(x−a)−cos(x−b)sin(x−a)cos(x−a)cos(x−b)sin(a−b)
[∵sinAcosB−sinBcosA=sin(A−B)]
=sin(x−b−x+a)cos(x−a)cos(x−b)sin(a−b)
=1cos(x−a)cos(x−b)
Hence proved.