What is the output of two cosine functions?

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Discussion Overview

The discussion revolves around the mathematical identity involving the sum of two cosine functions, specifically x1(t) = cos(ω1t) and x2(t) = cos(ω2t). Participants explore whether the output g(t) can be expressed as a sum and attempt to verify the identity through examples and counterexamples.

Discussion Character

  • Homework-related
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the problem of showing that g(t) = 0.5cos[(ω2-ω1)t] + 0.5cos[(ω2+ω1)t] and expresses difficulty in reaching a solution due to the complexity of trigonometric identities.
  • Another participant suggests testing the identity with specific values for ω1 and ω2, proposing a counterexample to check if the identity holds.
  • A participant provides specific values (ω1 = 30, ω2 = 60, t = 1) and calculates results that suggest the identity may not hold, raising questions about the original problem statement.
  • Clarification is sought regarding the notation used in the original question, particularly whether X1' refers to X2, which leads to confusion about the problem's intent.
  • One participant points out that the circled X in the original question represents a product, not a sum, which shifts the understanding of the problem.
  • Another participant reflects on the realization that the original question indicates a product, expressing relief and gratitude for the clarification.

Areas of Agreement / Disagreement

Participants express uncertainty about the identity's validity and whether the original problem statement was correctly interpreted. There is no consensus on the correctness of the identity, and multiple interpretations of the problem remain.

Contextual Notes

Participants note potential confusion stemming from the notation used in the original question, particularly regarding the representation of the functions and the operations involved (sum vs. product).

Jimmy Johnson
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Homework Statement


inputs x1(t) = cos(ω1t), x2(t) = cos(ω2t).
Show that output g(t) (sum of x1 + x2) = 0.5cos[(ω2-ω1)t] + 0.5cos[(ω2+ω1)t]

Homework Equations


included in upload of attempted solution. Trig identities.

The Attempt at a Solution



Uploaded in pdf. A lot more has been done on the solution however including it all would have taken forever, problem is I keep circling round and winding up back at the same place I started (due to the nature of the identities). It's been a while since I've did a question like this and understand half the battle is spotting the connection, and will probably be kicking myself if I get it done. Any help would be greatly appreciated.
 

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It doesn't look like a true identity to me. Try a disproof by counterexample. Choose a couple of representative values for the ω's and a handy value of t (say, t = 1). Does the identity hold?
 
You seem to have a point. I did ω1 = 30 ,ω2 = 60 t = 1. The given proof that I am to aim towards gave (√3)/4 while adding the two functions separately gave ((1+√3)/2).
ω1 = 30 ,ω2 = 60 t = 1... proof = ((-√6+√2)/4) as opposed to ((-√3+√2)/2). So it would seem the identity doesn't hold?

In the question it says X1 = cos(ω1t), X1' = cos(ω2t). I assumed from the layout of the diagram (the circled x indicating a sum) and the two inputs of X1 and X2 pointing logically towards a typo and X1' (no dash in question only for indicative purposes writing this) being X2 in connection with ω2 would make sense.

That being said if they were to be both of X1, assuming it wasn't a typo, would that make a difference? I would be confused as to the purpose of X2 if it were the case and how it would be any different under those circumstances anyhow. Unless it may then fall under the multiple angle formula and use recurrence relationships?
 
Can you post the original question? Perhaps a picture? It's difficult to make out what the situation is without context.

Edit: Nevermind. I just realized that you did post the question in your pdf. Sorry about that.
 
the original question is in the pdf attatched? or is it difficult to make out?
 
The circled X represents a product, not a sum.
 
wow. Well that will hopefully clear it up, I've to head to work for a few hours here then i'll get back to the drawing board. In the question it says that it can be shown as a sum? Would that not indicate that it should be proven as a sum i.e. the two x terms added and that my working out with the counter examples may be changed and hold?

I can't do much on it now but hopefully that will sort it out. Thanks a lot :)
 
Jimmy Johnson said:
In the question it says that it can be shown as a sum? Would that not indicate that it should be proven as a sum i.e. the two x terms added and that my working out with the counter examples may be changed and hold?
No, the two x terms are multiplied, but the result can be expressed as a sum.
 
Yeah I got it pretty much instantly after it being pointed out to be a product. Fell pretty dumb haha Thanks for the help!
 

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