What is the Sum of a Finite Sine Series?

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Discussion Overview

The discussion revolves around the sum of a finite series of sine functions, specifically the expression \(\sum_{n=1}^N \sin^2 (n \phi)\). Participants are exploring methods to derive this sum and clarifying the variable \(\phi\).

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • One participant inquires about the sum of the series and mentions a text that suggests the sum is \((N+1)/2\), expressing difficulty in deriving this result.
  • Several participants ask for clarification on the variable \(\phi\), indicating a need for more context.
  • Another participant provides a definition for \(\phi\) as \(\pi s /(N+1)\), where \(s\) ranges from 1 to \(N\).
  • A suggestion is made to resolve \(\sin^2\) using the identity \(1 - \cos(2\theta)\) and to express the series in terms of complex exponentials, which could lead to a geometric series.

Areas of Agreement / Disagreement

Participants are seeking clarification and exploring different approaches, but there is no consensus on the derivation or the final result of the series sum.

Contextual Notes

Participants have not fully resolved the mathematical steps involved in deriving the sum, and the discussion includes assumptions about the definitions and identities used.

shehry1
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Can anyone tell me what is the sum of a finite series of Sines.

[tex]\sum_{n=1}^N \sin^2 (n \phi)[/tex]. I am going through a text and it gives it as (N+1)/2. I tried to derive it. The N comes out ok when you use the half angle identity but I can't figure out a general rule for the Cosines that appear
 
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What is [itex]\phi[/itex]?
 
HallsofIvy said:
What is [itex]\phi[/itex]?

[tex]\phi = \pi s /(N+1)[/tex] where s = 1,2...N
 
resolve sin^2 into 1-cos something. Put it in terms of complex exponentials. you'll get a geometric series.
 

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