Understanding Maclaurin Series & De Moivre's Theorem

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SUMMARY

The discussion focuses on deriving the Maclaurin series for the exponential function and its connection to De Moivre's theorem. Participants explore the infinite series representation of e^(iθ) and its relationship to cos(nθ). The series in question is identified as ∑_{n=0}^{∞} (2^n cos(nθ))/n!, which can be simplified using Euler's formula to relate it to the Maclaurin series of e^(2Re[e^(iθ)]). This establishes a clear link between the Maclaurin series and De Moivre's theorem.

PREREQUISITES
  • Understanding of Maclaurin series and Taylor expansions
  • Familiarity with Euler's formula: exp(ix) = cos(x) + i sin(x)
  • Basic knowledge of complex numbers and their properties
  • Experience with infinite series and convergence
NEXT STEPS
  • Study the derivation of the Maclaurin series for e^x, sin x, and cos x
  • Learn about the applications of De Moivre's theorem in complex analysis
  • Explore the relationship between complex exponentials and trigonometric functions
  • Investigate convergence criteria for infinite series
USEFUL FOR

Students and educators in mathematics, particularly those studying calculus, complex analysis, and series expansions. This discussion is beneficial for anyone looking to deepen their understanding of Maclaurin series and their applications in proving mathematical theorems.

henryc09
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Homework Statement


[PLAIN]http://img263.imageshack.us/img263/9336/seriesgay.jpg

In the previous part of the question we had to show where the taylor expansion comes from, and calculated the maclaurin series for e^x, sin x and cos x. From that we had to prove De Moivre's theorem and so I would imagine that these things help in the last part of this question. I can see it looks like a Maclaurin series, just not sure where to start really.

Any help would be appreciated, thanks.
 
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What have you tried? Does that infinite series sort of look like any other infinite series you know?
 
well it looks like a maclaurin series, but I don't really know how to work out what it's a Maclaurin series of.
 
Not really. A Maclaurin series has powers of x (or whatever the variable happens to be).

IOW, a Maclaurin series looks like this:
\sum_{n = 0}^{\infty} a_n x^n

Your series is
\sum_{n = 0}^{\infty} \frac{2^n~cos(n\theta)}{n!}
 
Yeah but I was thinking that it looked like something to do with e^(i\theta) to the power of n which would give terms of cos(n\theta). I'm not sure, if not how do I go about tackling the problem?
 
henryc09 said:
Yeah but I was thinking that it looked like something to do with e^(i\theta) to the power of n which would give terms of cos(n\theta). I'm not sure, if not how do I go about tackling the problem?

Hint: The real part of exp(i x) = ?
 
cos(x)... sorry it's been a long day! Still not sure how to use that to get any further.
 
henryc09 said:
cos(x)... sorry it's been a long day! Still not sure how to use that to get any further.

Replace all the cos(nx) by exp(i n x) in the summation and then take the real part of the summation.
 
ok, I think I have it.

Is it the Maclaurin series for

e^(2*Re[e^(i\theta)])

that seems to work I think :s, meaning that the sum is just what's written above right?
 
  • #10
or rather:

Re[e^(2*e^(i0))]
 
  • #11
henryc09 said:
or rather:

Re[e^(2*e^(i0))]

Yes, and now you can simply this using Euler's formula

exp(ix) = cos(x) + i sin(x)
 

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