Is This A Correct/Sufficient Proof

  • Thread starter Thread starter Sylvester Sly
  • Start date Start date
  • Tags Tags
    Proof
Click For Summary

Homework Help Overview

The original poster presents a problem comparing the values of cos(sin(x)) and sin(cos(x)), seeking to determine which is greater through proof. The context involves concepts from trigonometry and function composition.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the validity of the original poster's assertion regarding function composition, specifically questioning the claim that f(g(x)) equals g(f(x)). There is mention of a proof by contradiction approach, with some participants noting the need to properly frame assumptions.

Discussion Status

The discussion is ongoing, with participants providing feedback on the original poster's approach and raising concerns about the logical structure of the proof. There is no explicit consensus on the correctness of the proof or the methods being employed.

Contextual Notes

Some participants highlight potential omissions in the original poster's statements and question the appropriateness of using proof by contradiction for the problem at hand. The nature of the problem and the assumptions being made are under scrutiny.

Sylvester Sly
Messages
2
Reaction score
0

Homework Statement



Which is greater cos(sin(x)) or sin(cos(x)), determine with proof.

The Attempt at a Solution



http://img21.imageshack.us/img21/5193/proof1ii.jpg
http://img109.imageshack.us/img109/8678/proof2w.jpg
 
Last edited by a moderator:
Physics news on Phys.org
Something you asserted early on is not true - that f(g(x)) = g(f(x)). I stopped reading after that.
 
Mark44 said:
Something you asserted early on is not true - that f(g(x)) = g(f(x)). I stopped reading after that.

I was doing proof by contradiction. In order to prove f(g(x)) =/= g(f(x)) i started off by letting f(g(x)) = g(f(x)) and working from there.
 
Sylvester Sly said:
I was doing proof by contradiction. In order to prove f(g(x)) =/= g(f(x)) i started off by letting f(g(x)) = g(f(x)) and working from there.
You would have been doing a proof by contradiction if you said you were doing a proof by contradiction. The way you've written it, you're just asserting (without justification) something that is not even true.

Other than that, it seems reasonable...
 
If you're doing a proof by contradiction, you don't start with "therefore ..." - You start by assuming the thing you want to contradict.

Also, in the same line you have "Therefore g(f(x)) = f(g(x)) cos(sin(x))". It looks like you omitted part of what you wanted to say.
 
Mark44 said:
If you're doing a proof by contradiction, you don't start with "therefore ..." - You start by assuming the thing you want to contradict.

Also, in the same line you have "Therefore g(f(x)) = f(g(x)) cos(sin(x))". It looks like you omitted part of what you wanted to say.
I assumed that was a typos for "g(f(x))= f(g(x))= cos(sin(x))". But, in any case, if the problem was to determine which was larger, I don't see what a proof by contradiction that they are not equal would accomplish.
 

Similar threads

  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 31 ·
2
Replies
31
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 18 ·
Replies
18
Views
2K
Replies
5
Views
2K
Replies
11
Views
7K
  • · Replies 4 ·
Replies
4
Views
3K