You are Here: Home >< Maths

# How do i prove this?

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
1. tan(3x)=3tanx-tan^3(x)/1-3tan^2x
2. (Original post by jeja12)
tan(3x)=3tanx-tan^3(x)/1-3tan^2x
Split into tan(2x + x).

That gives you (tan 2x + tan x) / (1 - tan 2x tan x)

now use tan 2x = 2 tan x / (1- tan^2 x).
3. thx
4. (Original post by jeja12)
thx
No problem.
5. Also how can I express cos4a in terms of cosA only.
6. (Original post by Zacken)
cos 4a = cos (2(2a)) = 2cos^2 2a - 1= 2(1-cos^2 a)^2 - 1
how did you factorise 2cos^2 2a -1 into that
7. (Original post by jeja12)
how did you factorise 2cos^2 2a -1 into that
What I wrote was wrong, I'll delete it, sorry.

What I meant was: cos 4a = cos (2(2a)) = 2cos^2 2a - 1

The cos 2a = 2cos^2 a - 1

So plug that in:

2 (cos 2a)^2 -1 becomes 2(2cos^2 a -1)^2 - 1
8. I get it now thank you very much
9. (Original post by jeja12)
I get it now thank you very much
No problem!

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: June 13, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read here first

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams