Proving Trig Statements: cos^2 + cos^4 + ... + cos^30 ≈ 15cos^11.38211

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Homework Help Overview

The discussion revolves around proving trigonometric statements involving sums of even powers of cosine, specifically the expressions cos²(θ) + cos⁴(θ) + ... + cos²⁰(θ) and cos²(θ) + cos⁴(θ) + ... + cos³⁰(θ), with the goal of approximating them with specific cosine powers.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the nature of the series and question how to prove the approximations. Some mention the geometric series and its properties, while others express uncertainty about the origins of the exponents in the approximations.

Discussion Status

The discussion is ongoing, with participants sharing their thoughts on the nature of the series and questioning the context of the problem. Some have suggested potential connections to geometric series, but there is no consensus on how to proceed or simplify the expressions further.

Contextual Notes

Participants have noted the constraints of the problem, specifically the range of θ being limited to |θ| ≤ π/2. There is also a mention of the original poster's personal discovery of the relationships while working on a related problem.

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Homework Statement



Show that [tex]cos^{2}(\theta)+cos^{4}(\theta)+cos^{6}(\theta)+cos^{8}(\theta)\approx 4cos^{4.3128}(\theta) , \mid\theta\mid\leq\pi/2[/tex]

and

[tex]cos^{2}(\theta)+cos^{4}+...+cos^{30}(\theta)\approx 15cos^{11.38211}(\theta) , \mid\theta\mid\leq\pi/2[/tex]

Homework Equations





The Attempt at a Solution



Any method?
 
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Well, I don't see how to prove it. Can you give us the context where this problem came from? Where did you find it?
 


1/(1-x) = 1+x+x^2+...

Isn't there a similar formula if the series terminates?
 


LCKurtz: I've actually came up with this question myself cos i saw this relationship for any sum of even power cosine while doing a problem ...

Berko: What do u exactly mean? Please enlighten me :)
 


Berko said:
1/(1-x) = 1+x+x^2+...

Isn't there a similar formula if the series terminates?

Sure, the problem is a geometric series. And

[tex]\sum_{k=1}^4 \cos^{2k}(\theta) = \frac{\cos^2(\theta)-\cos^{10}(\theta)}{1-\cos^2(\theta)}[/tex]

How does that help?
 


Yes! I do recognise that this is a geometric series but nevertheless this cannot lead us to further simplification...
 


hmm yah I simplified somewhat but not sure where those exponents come from.
 

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