# Math Help - Proving a Trig Identity

1. ## Proving a Trig Identity

Hi there, I need help proving a trig ident. steps plz,

cos2x= (1-tan^2(x))
..........(1+tan^2(x))

tan^2(x) is tan(x)^2....if u don't understand my writing

Please, any help is appreciated, thanks

2. You can use the identity:

$tan^{2}(x)=\frac{1-cos(2x)}{1+cos(2x)}$

It'll fall into place.

3. thanks so much

4. Originally Posted by compuwiz
cos2x= (1-tan^2(x))
..........(1+tan^2(x))
$1+\tan^2x=\sec^2x.$ (Basic identity.)

Now split the original fraction into two sums:

$\frac{{1 - \tan ^2 x}}
{{\sec ^2 x}} = \cos ^2 x - \sin ^2 x.$

And this is exactly equal to double-angle cosine formula.