x Turn on thread page Beta
 You are Here: Home >< Maths

# C3 Trig - Compound angles proof watch

1. https://onedrive.live.com/redir?resi...nt=photo%2cJPG

I don't get how you get from the second last line to the last line.

Can someone explain?

Remember that

3. Not a fan of geometric proofs. Prefer using the fact that
and
and then do it algebraically.
4. (Original post by B_9710)
Not a fan of geometric proofs. Prefer using the fact that
and
and then do it algebraically.
But then your point is invalid because those equations rely on the compound angle formula.
5. (Original post by MathsAndChess)
But then your point is invalid because those equations rely on the compound angle formula.
No they don't.
6. (Original post by B_9710)
No they don't.
How do you prove that statement then?
7. (Original post by MathsAndChess)
How do you prove that statement then?
Complex numbers.
8. (Original post by B_9710)
Complex numbers.
I'm amusing you mean though the fact
9. (Original post by MathsAndChess)
I'm amusing you mean though the fact
Depends how you define sin and cos.
10. I guess if you're speaking from a geometric definition of sin and cos. Which I am then my statement is true. Else then I see why you wouldn't require this.
11. (Original post by MathsAndChess)
I guess if you're speaking from a geometric definition of sin and cos. Which I am then my statement is true. Else then I see why you wouldn't require this.
If you use the definitions above and find you will find that by the definition above that this is the same as
You can find other results similarly.
12. (Original post by MathsAndChess)
I guess if you're speaking from a geometric definition of sin and cos. Which I am then my statement is true. Else then I see why you wouldn't require this.
Yeah if you use the geometric definitions you need to derive the double angle formulae before you can do anything interesting.

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:
Updated: March 2, 2016
Today on TSR

### Loughborough better than Cambridge

Loughborough at number one

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams