# Math Help - trig fix

1. ## trig fix

$\frac{1+tan^2[\frac{\pi}4-a]}{1-tan^2[\frac{\pi}4-a]}=cos 2A$ Prove this

2. ## Re: trig fix

Assuming a = A?

Have you tried to expand the LHS out with the compound angle formula for tan?

Recall $\displaystyle \tan (a\pm b)= \frac{\tan a \pm \tan b}{1\mp \tan a \tan b}$

3. ## Re: trig fix

I tried this way but finally i get 1/sin2a.

4. ## Re: trig fix

Originally Posted by srirahulan
$\frac{1+tan^2[\frac{\pi}4-a]}{1-tan^2[\frac{\pi}4-a]}=cos 2A$ Prove this
As written that statement is false.
To see that let $a=\frac{\pi}{4}$

If you change $\cos(2a)$ to $\csc(2a)$ then it is true.

5. ## Re: trig fix

yes,I can understand.