# Double angle formulae

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• October 3rd 2011, 12:31 PM
sakuraxkisu
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! :)
• October 3rd 2011, 02:42 PM
skeeter
Re: Double angle formulae
Quote:

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 ...
• October 5th 2011, 01:41 PM
sakuraxkisu
Re: Double angle formulae
Thank you so much! I feel like a bit of an idiot as that was so simple haha :)