Differentiating parametric equations

Hi,

I've been working on a problem in my textbook and have got stuck. The problem is:

A curve is defined by the parametric equations

$x = sec\theta + tan \theta$,
$y = sec \theta - tan \theta$,

where $\theta$ is a parameter.

Show that $\frac{dy}{dx} = \frac{sin\theta -1}{sin\theta +1}$

I can see that $\frac{dy}{dx} = \frac{sec\theta tan\theta - sec^2\theta}{sec\theta tan\theta + sec^2\theta}$ but I can see no way of arriving at $\frac{sin\theta -1}{sin\theta +1}$

Could anybody give me an idea of how to do this?

Multiply top and bottom by $\cos^2 \theta$

Oh, thanks BabyMaths...