Thread: 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)

No...

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

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

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)

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. is not the same as tan(2x), so if tan(x) = 2 then . 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 , you want to use 4. Make sure you understand this.

-Dan