# cosine of a double angle

• April 22nd 2009, 02:48 PM
cosine of a double angle
if cos θ=a, express 2θ in terms of a.
• April 22nd 2009, 03:00 PM
e^(i*pi)
Quote:

Originally Posted by blame_canada100
if cos θ=a, express 2θ in terms of a.

Use the identity $cos(2\theta) = 2cos^2(\theta) - 1$

(this is derived from the double angle formula for cos and the Pythagorean identity, both of which you'll need to know)

Knowing $cos^2 (\theta) = (cos(\theta))^2 = a^2$ you can solve it