rectangular equation : x^2 - y^2 = 4
use the double angle forumla
i have no idea what to do?
Hello tomatoesUsing $\displaystyle x = r\cos\theta, y = r\sin\theta$, we get:
$\displaystyle r^2\cos^2\theta - r^2\sin^2\theta = 4$
$\displaystyle \Rightarrow r^2(\cos^2\theta - \sin^2\theta) = 4$
But, using the double-angle formula, $\displaystyle \cos^2\theta - \sin^2\theta = \cos2\theta$, this becomes
$\displaystyle r^2\cos2\theta = 4$
or $\displaystyle r^2 = 4\sec2\theta$
Grandad