Question

find cosA.cos(60-A).cos(60+A)

cos3A

4cos3A

4sin3A

1/4(cos3A)​

Answer 1

answer is in this image.

Answer 2

Answer:

Step-by-step explanation:

here ,

cos (60-A) cos (60+A) = cos^2(A)- sin^2 (60)

= cos ^2 (A) - 3/4

now ,

cos A cos (60-A) cos (60+A = cos A ( cos^2 (A) - 3/4)

\ = 1/4 ( 4 cos ^3 (a) - 3 cos A)

{since 4 cos ^3 (a) - 3 cos A = cos 3A }

= 1/4 ( cos 3A)

hence proved