Cos^4a=1/8(3+4cos2a+cos4a)

Below.

Step-by-step explanation:

cos2a = 2 cos^2 a - 1

cos^4a =  8cos^4 a - 8cos^2 a + 1

Right side of the identity =

1 /8(3 + 4cos2a + cos4a)

= 1/8( 3 + 4(2 cos^2 a - 1) + 8cos^4 a - 8cos^2 a+1)

= 1/8 (3 + 8 cos^2 a - 4 + 8 cos^4 a - 8cos^2 a + 1)

= 1/8 (8cos^4 a)

= cos^4 a = the left side.

I realised that I should have derived the identity for cos 4a as well , which i have done in the picture.

idk

step-by-step explanation: