A Generalized trigonometric identity for Cos(x_1++x_n)?

Click For Summary

Discussion Overview

The discussion revolves around the existence of explicit formulas for the cosine and sine of the sum of multiple angles, specifically Cos(x_1 + x_2 + ... + x_n) and Sin(x_1 + x_2 + ... + x_n). Participants explore whether these can be expressed as sums of products of Sin(x_i) and Cos(x_i) without repeated application of the Cos(A+B) and Sin(A+B) identities.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Naturepaper questions if there is an explicit formula for Cos(x_1 + x_2 + ... + x_n) and Sin(x_1 + x_2 + ... + x_n), suggesting that expanding using Cos(A+B) and Sin(A+B) repeatedly may be necessary.
  • Some participants reference existing trigonometric identities, noting that the case where all angles are equal can be addressed with the multiple angle formula.
  • One participant points to a specific identity involving secants that relates to the cosine of the sum of angles, but acknowledges it may not directly answer Naturepaper's question.
  • Naturepaper proposes an intuitive expression for Sin(x_1 + x_2 + ... + x_n) involving products of sine and cosine terms, indicating a desire for a formula that converts products of these terms into sums.
  • Naturepaper later claims to have derived an expression for Sin(x_1 + x_2 + ... + x_n) and expresses intent to provide a proof and detailed formula.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of a general formula for the sums of sine and cosine terms. Multiple viewpoints and approaches are presented, with some participants suggesting potential methods while others express uncertainty about the feasibility of finding such formulas.

Contextual Notes

Some participants highlight the complexity of deriving a general formula, noting that existing identities may not directly apply to the finite sums in question. There is also mention of the need for further exploration and potential proofs.

NaturePaper
Messages
70
Reaction score
0
Hi Everyone,
Do there exist any explicit formula for Cos(x_1+x_2+...+x_n) as a sum of products of Sin(x_i) & Cos(x_i)? Or we need to expand using Cos(A+B), Sin(A+B) again & again?
If it exists then what is about Sin(x_1+x_2+...+x_n)?

[It is understood that there will be 2^(n-1) number of terms in the sum each of which will be
a product of n number of Sin(x_i) &/ Cos(x_i)]

Regards
Naturepaper
 
Mathematics news on Phys.org
http://en.wikipedia.org/wiki/Trigonometric_identity

That link has some things that are related, but not exactly the same.

I'm not sure if a formula can be found, but you can try it. Expand it for a couple of cases, try to find a formula and use induction.
 
Thank you Gib Z for your reply. But the link you given doesn't answer the question. It was for infinite sum whereas I need for finite terms. Anyway I'm waiting for more reply...Cheers
NaturePaper
 
NaturePaper, the same page gives
[tex]\sec(\theta_1 + \cdots + \theta_n) = \frac{\sec\theta_1 \cdots \sec\theta_n}{e_0 - e_2 + e_4 - \cdots}.[/tex]
From that, it's obvious that
[tex]\cos(\theta_1 + \cdots + \theta_n) = \frac{e_0 - e_2 + e_4 - \cdots}{\sec\theta_1 \cdots \sec\theta_n}.[/tex]
 
@adriank,
oh, very nice-I missed it. Thanks.

Intuitively, Sin(x_1+x_2+...+x_n)=sinx_1.sinx_2...sinx_n(e_1-e_3+e_4-...), where e_i are in terms of cotx_i. I'll check it.

Actually I need a formula to convert the product of K number of sinx_i and (n-k) number of cosx_i into sum of Sin and cosine terms.

If I get the expressions for Sin(x_1+x_2+...+x_n) and cos(x_1+x_2+...+x_n), after some manipulation I can get the required formula.

Thanks & Regards
NaturePaper
 
Got it.

Sin(x_1+x_2+...+x_n)=cosx_1cosx_2...cosx_n(e_1-e_3+e_4-...).

By the way, I'll post a possible proof and detailed formula soon.

cheers,
NaturePaper
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 11 ·
Replies
11
Views
2K
  • · Replies 25 ·
Replies
25
Views
4K
  • · Replies 40 ·
2
Replies
40
Views
5K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 24 ·
Replies
24
Views
4K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 125 ·
5
Replies
125
Views
20K