Let us consider the LHS
sin2 42° – cos2 78° = sin2 (90° – 48°) – cos2 (90° – 12°)
= cos2 48° – sin2 12° [since, sin (90 – A) = cos A and cos (90 – A) = sin A]
As we know, cos (A + B) cos (A – B) = cos2A – sin2B
Now the above equation becomes,
= cos2 (48° + 12°) cos (48° – 12°)
= cos 60° cos 36° [since, cos 36° = (√5 + 1)/4]
= 1/2 × (√5 + 1)/4
= (√5 + 1)/8
= RHS
Thus proved.