# Thread: Proving an identify with tan in it

1. ## Proving an identify with tan in it

It's me again and I've got another problem that I've been working on for two days, and I just can't get started on it!

Prove the identity (1 - tan^2x ) / (1 + tan^2x) = 1 - 2sin^2x.

So what I have so far is:

(1 - tan^2x) / (sec^2x)

Now what I do next?

2. ## Re: Proving an identify with tan in it

Consider writing:

$\displaystyle \frac{1-\tan^2x}{\sec^2x}=\frac{1}{\sec^2x}-\frac{\tan^2x}{\sec^2x}$

Now rewrite the two terms, and then apply a Pythagorean identity.

3. ## Re: Proving an identify with tan in it

Ok, I understood what you did so far, but now to 'rewrite the terms' has me totally confused. I know the Pythagorean identities by heart, but so far I don't see anywhere I can apply it. Thanks for the patience!

4. ## Re: Proving an identify with tan in it

Since we have:

$\displaystyle \sec(x)\equiv\frac{1}{\cos(x)}$ we may write:

$\displaystyle \frac{1}{\sec^2(x)}=\cos^2(x)$

$\displaystyle \frac{\tan^2(x)}{\sec^2(x)}=\tan^2(x)\cos^2(x)= \sin^2(x)$

5. ## Re: Proving an identify with tan in it

Ok, so now the Pythagorean identity is cos^2x + sin^2x = 1.

What we have so far is cos^2 - sin^2, yes? Now... I feel so dumb and stupid, I must have some mental deficiency but I just can't seem to relate the two. Please show me how, and I'll study it till I get it.

6. ## Re: Proving an identify with tan in it

Yes, you now have:

$\displaystyle \cos^2(x)-\sin^2(x)$

Now, use $\displaystyle \cos^2(x)=1-\sin^2(x)$ to write:

$\displaystyle 1-\sin^2(x)-\sin^2(x)=1-2\sin^2(x)$

And you are done.

7. ## Re: Proving an identify with tan in it

Ah, it's just a question of logic, isnt it? I don't know why I have none. Well, just gotta work at it! Thank you very much, I understand it now, but I'll go through this example over and over.

So, all I need to know is these 2 rules:

tanx = sinx / cosx
cos^2x + sin^2x = 1

And then, just apply it everywhere. Ok.

8. ## Re: Proving an identify with tan in it

Just wanted to say, I went and did another past paper question just now on the same thing, and got the right answer now that I know how. Thanks for helping me understand!

9. ## Re: Proving an identify with tan in it

Another update for all my avid readers: just did two more examples, and got them quick and easy! I don't know what happened, but it seems I suddenly acquired the wonderful skill of logic.

I won't post again, I just wanted to express my thanks for the helpfulness of this board.