if cos θ=a, express 2θ in terms of a.

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- Apr 22nd 2009, 02:48 PMblame_canada100cosine of a double angle
if cos θ=a, express 2θ in terms of a.

- Apr 22nd 2009, 03:00 PMe^(i*pi)
Use the identity $\displaystyle cos(2\theta) = 2cos^2(\theta) - 1$

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

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