You are Here: Home >< Maths

# De Moivre's theorem watch

1. Need help with q3. I cant seem to do the first part. Here is what I have tried to do.
http://m.imgur.com/laIkAiy,0wlahrY,jriufqx
Thanks.
2. (Original post by Super199)
Need help with q3. I cant seem to do the first part. Here is what I have tried to do.
http://m.imgur.com/laIkAiy,0wlahrY,jriufqx
Thanks.
What's the question?
3. (Original post by Super199)
Need help with q3. I cant seem to do the first part. Here is what I have tried to do.
http://m.imgur.com/laIkAiy,0wlahrY,jriufqx
Thanks.
You shouldn't have rationalised the thingy, you should have done:

and then made use of the fact that and
4. (Original post by Zacken)
You shouldn't have rationalised the thingy, you should have done:

and then made use of the fact that and
But havent you rationalized it but just not using w but the original thing.
Do i plug in the things first or multiply everything?
5. (Original post by Super199)
But havent you rationalized it but just not using w but the original thing
Yes, but you rationalise things by multiplying by its conjugate. is not the conjugate of since , the conjugate is only the sign before the flipped.
6. (Original post by Zacken)
Yes, but you rationalise things by multiplying by its conjugate. is not the conjugate of since , the conjugate is only the sign before the flipped.
I see. Is it better to multiply things first or sub in the trig results.
7. (Original post by Super199)
I see. Is it better to multiply things first or sub in the trig results.
The trig identities he used are only applicable for the product.
8. I think this question would be easier if you used the exponential form of the complex number, w.
9. If then

So you get

If you divide through top and bottom by you get

.
10. (Original post by B_9710)
If then

So you get

If you divide through top and bottom by you get

.
Sorry I don't see where you are going with this?
11. (Original post by Super199)
Sorry I don't see where you are going with this?
Are you aware that

And .
12. (Original post by B_9710)
Are you aware that

And .
Nope. Where do these come from?
13. (Original post by Super199)
Nope. Where do these come from?
These are a direct result of Euler's formula: .
14. (Original post by Super199)
Nope. Where do these come from?
Well and you just use De Movire's theorem to prove the 2 results.

.
15. (Original post by B_9710)
Well and you just use De Movire's theorem to prove the 2 results.

.
Hmm i keep ****ing this up.
The top gives 2cos1/2x
The bottom gives -2isin1/2x?
Is that right?
16. (Original post by Super199)
Hmm i keep ****ing this up.
The top gives 2cos1/2x
The bottom gives -2isin1/2x?
Is that right?
Yes the 2 on top and bottom cancel. Then just move the imaginary part from denominator to numerator - how do yo think you do this?
17. (Original post by B_9710)
Yes the 2 on top and bottom cancel. Then just move the imaginary part from denominator to numerator - how do yo think you do this?
Rationalize?
18. (Original post by Super199)
Rationalize?
Exactly.

### Related university courses

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 13, 2016
Today on TSR

### Edexcel C4 Maths Unofficial Markscheme

Find out how you've done here

### 1,800

students online now

Exam discussions

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