You are Here: Home >< Maths

Quick Proof questions Watch

1. You are given that is a non-real root of the above equation, show that:

So I go:

3 roots are

And

Thus the sum of the roots must equal the coefficient of the which is 0 and so:

Is that good enough ? Any other ways ?
2. I assume you mean 1 + w + w^2 = 0?

The sum of the roots is acceptable I believe, but only if you have proved that 1, w and w^2 are the roots already. (which you did by saying what the 3 roots are, but you might have to do a little more to show that they are in fact the 3 roots of the equation)

Another way is to factorise z^3-1=0
into (z-1)(z^2+z+1) = 0 (you can figure this out the first time through polynomial division and then I think you can quote the result, at least you can in my system)
But w is non-real so w=/=1 so 1 + w + w^2 = 0
3. (Original post by Aeonstorm)
I assume you mean 1 + w + w^2 = 0?

The sum of the roots is acceptable I believe, but only if you have proved that 1, w and w^2 are the roots already. (which you did by saying what the 3 roots are, but you might have to do a little more to show that they are in fact the 3 roots of the equation)

Another way is to factorise z^3-1=0
into (z-1)(z^2+z+1) = 0 (you can figure this out the first time through polynomial division and then I think you can quote the result, at least you can in my system)
But w is non-real so w=/=1 so 1 + w + w^2 = 0
Ah yea, After finding the 3 roots I would let one non-real root be and then show that and so is also a root and then proceed as I did above.

I probably wouldn't have spotted your method and it would take too long for me to divide the thing by
4. (Original post by member910132)
Ah yea, After finding the 3 roots I would let one non-real root be and then show that and so is also a root and then proceed as I did above.

I probably wouldn't have spotted your method and it would take too long for me to divide the thing by
I recommend you ask your teacher whether you are allowed to quote the general result:

z^n -1 = (z-1)(z^(n-1) + z^(n-2) + z^(n-3) + ... + 1)

rather than actually deriving it each time. If you are allowed to, it is a slight bit quicker than showing that the roots are 1, w and w^2 and then using the sum of the roots. If not, your method is perfectly valid.
5. (Original post by Aeonstorm)
I recommend you ask your teacher whether you are allowed to quote the general result:

z^n -1 = (z-1)(z^(n-1) + z^(n-2) + z^(n-3) + ... + 1)

rather than actually deriving it each time. If you are allowed to, it is a slight bit quicker than showing that the roots are 1, w and w^2 and then using the sum of the roots. If not, your method is perfectly valid.
Self teaching FM lol, that's why I post about 3-4 questions in here a day.
6. (Original post by member910132)
Self teaching FM lol, that's why I post about 3-4 questions in here a day.
I also used this forum regularly when I self-taught FM. The combination of my textbooks and this forum was all I really needed.

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: May 5, 2012
Today on TSR

Half of the top 10 aren't even RG...

My alarm went off in an exam!

Discussions on TSR

• Latest
• See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• 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

Chat with other maths applicants

Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE