# Double Angle Formulae

• May 1st 2011, 01:57 AM
johnsy123
Double Angle Formulae
If tan(x)=2 and x is element of [0, pi/2] find the values of

a)tan(2x)

b) sin(2x)

c) cos(2x)

This is what i did.
Since tan(x)=2, that would mean tan(2x)=4, so therefore tan(2x)=2tan(2)/1-tan(4)
• May 1st 2011, 02:41 AM
Prove It
No...

tan(2x) = 2tan(x)/{1 - [tan(x)]^2}.

You know that tan(x) = 2, so what is tan(2x)?
• May 1st 2011, 03:03 AM
topsquark
Quote:

Originally Posted by johnsy123
This is what i did.
Since tan(x)=2, that would mean tan(2x)=4, so therefore tan(2x)=2tan(2)/1-tan(4)

Quote:

Originally Posted by Prove It
No...

tan(2x) = 2tan(x)/{1 - [tan(x)]^2}.

You know that tan(x) = 2, so what is tan(2x)?

Further hint:

johnsy123: Your notation is terrible. $tan^2(x)$ is not the same as tan(2x), so if tan(x) = 2 then $tan^2(x) = 4$. However your intention is correct in how you are trying to put it into the tan(2x) formula. Instead of tan(4), which is also not equal to $tan^2(x)$, you want to use 4. Make sure you understand this.

-Dan