Question

Cosx cosx/2 - cos3x cos 9x/2 = sin 7x sin8x ​

Answer 1

Answer:

Step-by-step explanation:

cos x cos (x/2) - cos 3x cos(9x/2)

= sin 4x sin (7x/2)

Take L.H.S.: cos x cos x 2 x2- cos 3x cos 9 x 2 9x2  

Multiplying & Dividing by 2 : =  1 2 12{2cos x cos x 2 x2- cos 9 x 2 9x2 cos 3x} {∵ 2 cos A cos B = cos (A + B) + cos (A – B)} = sin 4x sin 7 x 2 7x2 = R.H.S.  Hence Proved