Originally Posted by

**Lloyd78** Hello,

So I want to evaluate cos(2x) in c. Using Trigonometric Identities:

cos(2x) = cos^2(x) - sin^2(x) = 2 cos^2(x) - 1 = 1 - 2 sin^2(x)

Where:

cos(2x) = 1 - 2 sin^2(x)

However:

sin^2(x) = 1/2 - 1/2 cos(2x)

So is this like the chicken and the egg?

Using sin^2(x) to evaluate cos(2x) which needs sin^2(x). I only have access to the basic functions cos, sin etc and I was hoping to evaluate cos(2x) using a combination of these.

Or is cos(2x) simply the cosine of twice the angle. I have only recently returned from a long hiatus from the world of mathematics so please forgive me if the solution is glaringly obvious. Any help would be greatly appreciated.

Cheers,

Lloyd.