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I don't need reasons, just the steps.

Cos2x=(1-tan^2x/1+tan^2x)

Cos2x=(1-tan^2x/1+tan^2x)

## Comments

If you can't get it to a format you can read, your loss

Also, I've taken the double-angle identity for granted; I'd hope you'd remember that one by now.

\begin{align*}

\frac{1-\tan^2x}{1+\tan^2x} &= \frac{1-\sin^2x/\cos^2x}{1+\sin^2x/\cos^2x} \\

&= \frac{\cos^2x-\sin^2x}{\cos^2x+\sin^2x} \\

&= \cos^2x-\sin^2x \\

&= \cos2x \\

\end{align*}

At least this way he'll learn some LaTeX I hope.

It would probably be funny if you had a specific post of mine to show me.

Also, I guess it depends on one's mood at the time.

Though looking back you really did try to help. I was just to thick-headed to get it.. ^^ The sad think is the job I had to do this for was happy with a much easier solution.

CodeCogs LaTex Equation Editor

LaTeX is cool, and far more portable than any other equation format.