# Math Help - Double angle formulae

1. ## Double angle formulae

Hi, I have a question on double angle formulae:
Eliminate theta from the equation
x = 2 sec theta, y = cos 2 theta

The answer is x^2y = 8 - x^2

Thank you!

2. ## Re: Double angle formulae

Originally Posted by sakuraxkisu
Hi, I have a question on double angle formulae:
Eliminate theta from the equation
x = 2 sec theta, y = cos 2 theta

The answer is x^2y = 8 - x^2
$x = 2 \sec{\theta} \implies \cos{\theta} = \frac{2}{x}$

$y = \cos(2\theta) = 2\cos^2{\theta} - 1$

proceed ...

3. ## Re: Double angle formulae

Thank you so much! I feel like a bit of an idiot as that was so simple haha