(i) L.H.S.
= cos 2A + cos 2B – cos 2C
= (cos 2A + cos 2B) – cos 2C
= 2 cos (A + B) cos (A – B) – cos 2C
= 2 cos (180° – C) cos (A – B) – (2 cos2 C – 1)
= -2 cos C cos (A – B) -2 cos2 C + 1
= 1 – 2 cos C cos (A – B) -2 cos2 C
= 1 – 2 cos C [cos (A – B) + cos C]