Q6 of 90 Page 8

Prove that :

cos 3A + cos 5A + cos 7A + cos 15A= 4 cos 4A cos 5A cos 6A

Take L.H.S.


cos 3A + cos 5A + cos 7A + cos 15A


= (cos 5A + cos 3A) + (cos 15A + cos 7A)






= 2 cos 4A cos A + 2 cos 11A cos 4A


= 2 cos 4A (cos 11A + cos A)






= 4 cos 4A cos 5A cos 6A


= R.H.S.


Hence Proved


More from this chapter

All 90 →
4

Prove that:

i.


ii.

5

Prove that:

i.


ii. sin 47° + cos 77° = cos 17°

 6

Prove that :

cos A + cos 3A + cos 5A + cos 7A = 4 cos A cos 2A cos 4A

 6

Prove that :