Year 12s: what will be the first way you research your future options? >>

de moivre's trig series help

hi1345

Hi, can somebody explain how to do pt. 3 and 4 of this question? Thanks.

IMG_20140201_001939.jpg401.0KB

Reply 1

brianeverit

Original post by hi1345

Hi, can somebody explain how to do pt. 3 and 4 of this question? Thanks.

What have you done so far? I cannot help if I don't know where you are stuck and I'm not allowed to do it for you.

Reply 2

hi1345

Original post by brianeverit

What have you done so far? I cannot help if I don't know where you are stuck and I'm not allowed to do it for you.

I've done the other three parts now (not pt.3) and I can't get started on pt.3 as I dont know how to find the modulus as C+iS is a geometric series

Reply 3

brianeverit

Original post by hi1345

I've done the other three parts now (not pt.3) and I can't get started on pt.3 as I dont know how to find the modulus as C+iS is a geometric series

Can you see what the common ratio is? If so just use the formula for the sum of a G.P.

Reply 4

hi1345

Original post by brianeverit

Can you see what the common ratio is? If so just use the formula for the sum of a G.P.

I did but can't get sin(ntheta)/sin(theta)

Reply 5

user2020user

Original post by hi1345

I did but can't get sin(ntheta)/sin(theta)

Can you show us what you've got so far?

I've finished the question so I can help you out :smile:

Reply 6

hi1345

Original post by Khallil

Can you show us what you've got so far?

I've finished the question so I can help you out :smile:

Do I just use the geometric sum I'm just a bit confused about the modulus sings

Reply 7

hi1345

Original post by Khallil

Can you show us what you've got so far?

I've finished the question so I can help you out :smile:

This is what I got so far

Edit: do I just say part in front is modulus and so arg is ntheta?

(edited 10 years ago)

IMG_20140201_231713.jpg470.3KB

Reply 8

user2020user

Original post by hi1345

This is what I got so far

Edit: do I just say part in front is modulus and so arg is ntheta?

I'd say so.

Just state that

C+iS

is of the form

re^{i\phi}

, which means that:

\boxed{|C+iS| = r = \dfrac{\sin n\theta}{\sin \theta}}

and

\boxed{\arg \left( C + iS \right) = \phi = n\theta}

Hint

Further Hint

(edited 10 years ago)

Reply 9

hi1345

Original post by Khallil

I'd say so.

Just state that

C+iS

is of the form

re^{i\phi}

, which means that:

\boxed{|C+iS| = r = \dfrac{\sin n\theta}{\sin \theta}}

and

\boxed{\arg \left( C + iS \right) = \phi = n\theta}

Hint

Further Hint

Thank you, turns out I already had the answer then.

Quick Reply

Latest

Articles for you

What is a campus university? List of campus unis in the UK

University phone numbers and contact details for Ucas Clearing 2023

What is an LLM and how could it benefit your career?

Powerful questions business students forget to ask their career advisers

de moivre's trig series help

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you