# Math Help - Relationship between Cos2A and Cos4A

1. ## Relationship between Cos2A and Cos4A

Hi
How can these be expressed as the other?
So, waht is the relationship, in terms of identities, of cos4A to cos2A?

Thanks

2. Originally Posted by 200001
Hi
How can these be expressed as the other?
So, waht is the relationship, in terms of identities, of cos4A to cos2A?

Thanks
$\cos (4a) = \cos (2a + 2a)$. $\cos 2a$ is double angle and there are identities for that.