Talk:Law of cosines

message: The text quoted by Euclid concerning the geometric interpretation of the Law of Cosines is helpful but unclear in the sense that Euclid (or possibly Heath) did not name the thing Euclid meant to "namely" identify. In the quote Euclid states 'namely "that" on which the perpendicular falls', but it is unclear if by 'that' he means namely, 'the rectangle' or namely, 'the side about the obtuse angle' which he is describing. It seems to me either Euclid was not expressing himself clearly here or the translation by Heath is at fault but the end of the Euclid quote does not seem to describe a rectangle very well. It might have been clearer if Euclid had stated something closer to the following (CAPS mark my paraphrasing):
"Proposition 12
In obtuse-angled triangles the square on the side subtending the obtuse angle is greater than the EITHER OF THE squares on the sides containing the obtuse angle by twice the rectangle contained by one of the sides about the obtuse angle, namely that RECTANGLE on WHOSE LENGTH the SIDE SUBTENDING the perpendicular falls, and WHOSE WIDTH IS EQUAL TO the straight line SEGMENT cut off outside by the perpendicular towards the obtuse angle."
I am not suggesting that the quote be changed but only that a clarification be given by somebody more in the know on this than myself.
P.S. I only place this talk section above the others to more closely align it with the portion of the main article to which it pertains.
132.45.121.6 (talk) 17:48, 27 August 2012 (UTC)
thread_title:  Intended meaning of Euclid's Proposition should be made clear 

message: In "Using the distance formula" the article states as a fact that "We can place this triangle on the coordinate system by plotting..." and then proceeds to use some trigonometry in the coordinates, without any explanation for how those coordinates are derived / where they came from.
Just a comment about the readability- I can't actually see clearly the labels on Figure 2.  67.255.12.129 (talk) 20:52, 19 April 2011 (UTC)
thread_title:  Proof "Using the distance formula" unclear 

message: Okay, in the article, it states the since cos 90 is 0 the Law of Cosines is reduced to the Pythagorean Theorem. I don't think that's the case cause if a^2=b^2+c^2-2bccosA, then wouldn't a^2=b^2+c^2? —Preceding unsigned comment added by 70.107.165.59 (talk • contribs) 
:Yes, and that's the Pythagorean theorem (in the case where the right angles is the angle between the sides of lengths b and c). Michael Hardy 23:20, 1 March 2007 (UTC)
replies: text: The last time I checked, the Pythagorean theorem is a^2+b^2=c^2. I put something about this near the top of the page. Mason0190 (talk) 13:08, 6 May 2015 (UTC)
replies: text: 
It seems that your answer would technically be correct, Michael, but it's a bit unclear because the variables tend to be such that A and B are the legs and C is the hypotenuse. It's not a correctness issue, it's clarity. Mason0190 (talk) 13:10, 6 May 2015 (UTC)
thread_title:  Pythagorean Theorem also Proved by Law of Cosines? 

message: In the dot product article, a dot b = a b cos theta is proved using the Law of Cosines. And the Law of Cosines is proved using vector dot products. Shouldn't somebody fix this? --Orborde 07:48, 9 September 2005 (UTC)
replies: text: The dot product proof should be removed.  It's circular.  Law of cosines comes first historically and all of vector calculus is assumed to depend on it, not vice versa.  Might as well use vectors to prove the pythagorean theorem.  Pfalstad 10:22, 9 September 2005 (UTC)
replies: text:  I have hopefully solved this problem, by stating the law of cosines is equivalent to the dot product formula from theory of vectors. --345Kai 10:45, 30 March 2006 (UTC)

text:  "a dot b = a b cos theta" is the definition of a vector dot product.  No proof is not required. Jmheneghan (talk) 17:16, 20 May 2016 (UTC)
replies: text: Wow that's pretty darn smart. --M1ss1ontomars2k4 | T | C | @ 04:04, 21 May 2006 (UTC)
replies: text:  If you define the dot product (a,b)*(c,d) as ac+bd (so it is linear) and then prove (a,b)*(c,d)=|(a,b)|*|(c,d)|*cos(α) then there is nothing circular. The definition used is a matter of taste, I guess some others are possible. I like the argument with dot product because it gives a different light on the cosine law and I think it deserves a place on this article. Ricardo sandoval 23:20, 12 April 2007 (UTC)

text:  I never saw the story of the dot product but I would guess it came from the scalar product of physics (as in the definition of work done in a particle) is that right? Ricardo sandoval 23:20, 12 April 2007 (UTC)

text:  I guess we could also define (a,b)*(c,d) as |(a,b)|*|(c,d)|*cos(α) prove the linearity by geometrical arguments and use it to prove the cosine law. right? Ricardo sandoval 23:20, 12 April 2007 (UTC)
thread_title:  Circular proof? 

message: The Law of Cosines can also be used to find the measure of the three angles in a triangle if you know the length of the three sides. This is how you do it:
           
:a^2 + b^2 - 2ab\cos C = c^2
:          - 2ab\cos C = -a^2 - b^2 + c^2
:               \cos C = (-a^2 - b^2 + c^2) / (-2ab)
:               \cos C = (a^2 + b^2 - c^2) / (2ab)
:                    C = \arccos [(a^2 + b^2 - c^2) / (2ab)]
Now you can find the measure of angle C!
— Sverdrup (talk) 21:00, 25 Feb 2004 (UTC)
replies: text: As noted above, the vector-based proof is circular: the proof that the dot product of two vectors has a geometric interpretation in Euclidian space is itself based on the law of cosines.  So using the geometric interpretation of a dot product to prove the law of cosines is a bit problematic... --Delirium 03:17, 13 November 2005 (UTC)
replies: text: see above --345Kai 10:45, 30 March 2006 (UTC)
thread_title: Another use for Law of Cosines

message: Is the usage of "we" throughout this article proper? Shouldn't "we can easily prove" be "can be proved" (wlong with some sentence rearrangement). BrokenSegue 04:00, 30 March 2006 (UTC)
thread_title: We?

message: I have rewritten the article and expanded it considerably.  I have taken a lot of material from the French article, according to the above suggestion.  I hope you like it!!! Please improve further...
(and sorry that in the history of this page the big change was signed "Euklid".  That was me and and I know we shouldn't use these kinds of names on Wikipedia, so I've changed my login. --345Kai 10:45, 30 March 2006 (UTC)
thread_title: Rewrite

message: The last demonstrations using the power of a point can be simplified, all of them can be worked out with just one application of the power point theorem, and the the first two cases can be made one by considering b<c(always can be done by flipping the sides). Someone willing to make new pictures? Ricardo sandoval 08:02, 16 April 2007 (UTC)
thread_title:  Faster demonstrations

message: This proof along with that using Pythagoras theorem are of considerable historical significance since both have their origins in Euclid's Elements and both are referred to in the ground breaking work of Nicolaus Copernicus: "De Revolutionibus Orbium Coelestium". On Page 20 and Page 21 of Book 1 Copernicus describes two techniques for determining angles given all three sides of a triangle. These techniques correspond respectively to the Pythagorean and Power of Points derivations of the Law of Cosines.
Fig 1 and Fig 2 are replicas of the diagrams from Page 21 of De Revolutionibus Orbium Coelestium.In Fig 1 triangle ABC is drawn with B an acute angle. With C as centre and BC (=a) as radius semi-circle DBF is constructed with DF a diameter and D a point on AC. The circle meets side AB at E and EC is joined forming isosceles triangle ECB with EB = 2acos(B).Power of Points Theorem (intersecting secants theorem) is applied to point A outside the circle:
\begin{array}{lcl}
\quad\quad AD \times AF = AB \times AE\\
\Rightarrow (b-a)(b+a)=c(c-2a\cos\hat{B})\\
\Rightarrow \quad b^2-a^2=c^2-2ac.\cos\hat{B}\\
\Rightarrow \quad \quad b^2=a^2+c^2-2ac\cos\hat{B} 
\end{array}
It will be no different in the case of Fig 2 where angle B is obtuse:
\begin{array}{lcl}
\quad\quad AD \times AF = AB \times AE\\
\Rightarrow (b-a)(b+a)=c(c+2a\cos(180-\hat{B}))\\
\Rightarrow \quad b^2-a^2=c^2-2ac.\cos\hat{B}\\
\Rightarrow \quad \quad b^2=a^2+c^2-2ac\cos\hat{B} 
\end{array}
In addition to the two cases dealt with above, we also need to consider the situation shown in the diagram where Power of Point theorem is applied about point B inside the construction circle.
Triangle ABC is drawn with side AB=c,BC=a and CA=b. With A as centre construct circle DCE with radius b and diameter DE passing through B. CB produced meets the circle at F. Since triangle CAF is isosceles:
CF=2b\cos\hat{C}\quad.
Now apply the Power of a Point Theorem (intersecting chords theorem) to point B inside the circle:
\begin{array}{lcl}
\quad\quad BD \times BE = BC \times BF\\
\Rightarrow (b-c)(b+c)=a(2b\cos\hat{C}-a)\\
\Rightarrow \quad b^2-c^2=2ab.\cos\hat{C}-a^2\\
\Rightarrow \quad \quad c^2=b^2+a^2-2ab\cos\hat{C} 
\end{array}  —Preceding unsigned comment added by Neil Parker (talk • contribs) 10:28, 17 December 2007 (UTC)
thread_title:  Alternative proof using [http://www.cut-the-knot.org/pythagoras/PPower.shtml Power of a Point Theorem] 

message: I really like this proof, but it seems to be there are multiple cases.  All of which work out to the same correct result.
I will ignore cases when any of the angles are right angles,
as then the distance formula simply leads to the Pathagorian Theorem, which is a special case of the law of cosines.
The proof given works when angle at vertex C is acute and angle at vertex B is acute. Below are the other cases:
1) The angle at vertex C is acute, and the angle at vertex B is obtuse. a - b*cos(theta) becomes b*cos(theta) - a. (where as in the original theta is the angle at vertex C). Since the term is squared the result is the same since x**2 = (-x)**2
2) The angle at vertex C is obtuse. in this case the side: 
  a - b*cos(theta)  becomes a + b*cos(180-theta) which is
equal to a + b*(-cos(theta)) or a - b*cos(theta).  Again leading to the same result.
Does this seem reasonable to anyone?  —Preceding unsigned comment added by NeilAFraser (talk • contribs) 01:30, 12 July 2008 (UTC)
thread_title:  Distance Proof 

message: : 
\begin{align}
c^2 & = a^2 + b^2  - 2ab\cos \theta \\
 & = (a + b)^2  - 4ab\cos^2 \tfrac{\theta}{2}\\
 & = (a - b)^2  + 4ab\sin^2 \tfrac{\theta}{2}\\
\end{align}
The alternate forms were found after reading about rounding errors in spherical triangles, as mentioned in the great-circle distance article.
The arithmetic and geometric mean lengths of the known sides are:
:\begin{align}
m_a & = (a+b)/2\\
m_g & = \sqrt{a b}\\
\end{align}
And
: 
\begin{align}
c^2 & = (a + b)^2  - 4ab\cos^2 \tfrac{\theta}{2}\\
 & = 4 m^2_a  - 4 m^2_g \cos^2 \tfrac{\theta}{2}\\
 & = 4(m_a + m_g \cos\tfrac{\theta}{2}) (m_a - m_g \cos\tfrac{\theta}{2}) \\
 & = (2m_a + 2m_g \cos\tfrac{\theta}{2}) (2m_a - 2m_g \cos\tfrac{\theta}{2}) \\
\end{align}
Or
: 
\begin{align}
(\tfrac{c}{2})^2 & =  m^2_a  -  m^2_g \cos^2 \tfrac{\theta}{2}\\
 & = (m_a + m_g \cos\tfrac{\theta}{2}) (m_a - m_g \cos\tfrac{\theta}{2}) \\
\end{align}
- Ac44ck (talk) 23:40, 10 December 2008 (UTC)
thread_title:  Alternate forms 

message: I changed the line "CH = a cos(π – γ) = −a cos(γ)" in the history subsection to "CH = (CB) cos(π – γ) = −(CB) cos(γ)". In the equations used in this section and the diagram there is no "a" side. Danshil (talk) 21:39, 19 December 2009 (UTC)
thread_title:  Adjusted line "CH = a cos(π – γ) = −a cos(γ)" in history 

message: The diagram and symbols used for angles on the "Law of cosines" page are not the same as those on the "Law of sines" page.  Would it be reasonable to change this page to match those on the "Law of sines" page?  — Preceding unsigned comment added by 209.98.145.235 (talk) 14:36, 30 January 2013 (UTC)
thread_title:  Make consistent with "Law of sines" 

message: :\arccos\frac{1}{2.5}
:\arccos\frac{1}{2.5}
:\arccos\frac{4.25}{6.25}
consecutive numbers.
:1.5, 2.5, 3.5
:4.25, 5.25, 6.25
:\frac{1}{2.5}
:1.5\times3.5=5.25
1-0.68=0.32 change between the law of sin and the law of cosine
:\frac{\sin A}\frac{1}{2.5}=\sqrt5.25
:\frac{\sin B}\frac{1}{2.5}\sqrt5.25
:\frac{\sin C}\frac{1}{6.25/2}=\sqrt5.25
consecutive numbers.
:1.5, 2.5, 3.5
:4.25, 5.25, 6.25
:\frac{1}{2.5}
:1.5\times3.5=5.25  — Preceding unsigned comment added by 199.119.233.245 (talk) 19:12, 7 November 2015 (UTC)
thread_title: relation between the law of sin and cosine

message: In the section where you use the quadratic equation to make a form of the law of cosines that solves angle-side-side triangles, there is an error on the bounds checking.  You say that if c>=b, there are no solutions.  However:
- If c=b, there is one solution.  It is an isosceles triangle with b=c, B=C.  The triangle itself is twice the size as the right triangle formed by c=b*sin(C) & B = 90.  B and C are acute.
- If c>b, there is also one solution.  It is a scalene triangle with B and C being acute.  — Preceding unsigned comment added by 69.159.12.95 (talk) 01:35, 26 January 2020 (UTC)
replies: text: For future reference, WP:SOFIXIT applies.  –Deacon Vorbis (carbon • videos) 01:51, 26 January 2020 (UTC)
thread_title:  Correction for solving angle-side-side triangles 

message: Right now it says, "only one positive solution if c = b sin γ".  It should say "only one positive solution if c = b sin γ OR if c>=b"  — Preceding unsigned comment added by 69.159.12.95 (talk) 06:06, 26 January 2020 (UTC)
replies: text: WP:SOFIXIT.  Also, please place new comments at the end and sign your posts. See Help:Talk for more info. –Deacon Vorbis (carbon • videos) 12:44, 26 January 2020 (UTC)
thread_title:  Another small addition on the angle-side-side solution 

message: Will it make sense to add a demonstration with Chasles relation and scalar product?
Even if historically it was after, it is a simple and powerful tool to demonstrate it.  — Preceding unsigned comment added by 103.139.171.78 (talk) 17:49, 15 July 2020 (UTC)
thread_title:  Demonstration by Chasles relation and scalar product 

message: Could there be a valid proof using the definition of a dot product.
With triangle having side C as the vector A-B, where A,B are the vectors of the triangle legs: then length |C|^2 could be written as |A|^2+|B|^2-2A(dot)B. A(dot)B is defined as |A||B|cos(theta) giving the law of cosines formula. If other editors think this is valid, I could add a formatted proof. Nikolaih☎️📖 21:40, 6 August 2020 (UTC)
thread_title:  Proof by application of dot product 

message: Perhaps i'm mistaken but angle ABC in Fig 2 appears acute rather than obtuse to me. Proof reading anyone? — Preceding unsigned comment added by 198.24.255.98 (talk) 16:24, 22 September 2020 (UTC)
replies: text: No, it's definitely obtuse (unless there's some issue with it being displayed in a weird aspect ratio). –Deacon Vorbis (carbon • videos) 16:55, 22 September 2020 (UTC)
replies: text: The angle ABC in Fig 2 clearly looks acute on my monitor. Just to be clear an acute angle is smaller than 90 degrees and an obtuse angle is greater than 90 degrees but less than 180 degrees. It makes the whole section unreadable, and brings into question the correctness of articles on the site.  — Preceding unsigned comment added by 2605:A601:A706:8700:F91E:CE18:C1EC:7244 (talk) 23:19, 27 September 2020 (UTC)
replies: text: Oh, I think I see the confusion.   ABC is acute, but  ABC is obtuse, because angle ACB is an obtuse angle. –Deacon Vorbis (carbon • videos) 23:28, 27 September 2020 (UTC)
replies: text: Right, ABC is an angle not a triangle. 'Triangle ABC' is confusing if not complete nonsense.  — Preceding unsigned comment added by 198.24.255.98 (talk) 23:35, 2 October 2020 (UTC)
thread_title:  Requires proof reading?