Consider L.H.S. = cos 15° cos 35° cosec 55° cos 60° cosec 75°

= cos 15° cosec 75° cos 35° cosec 55° cos 60°

= cos 15° cosec (90-15)° cos 35° cosec (90-35)° cos 60°

= (cos 15° sec 15°) × (cos 35° sec 35°) × cos 60°

= (1) × (1) × (1/2)

= 1/2 = R.H.S.

Hence, proved.