Talk:List of trigonometric identities
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CPU Usage[edit]
This page, despite seeming to have no active content, utilizes 100% of the core the thread is running on. Google Crome Version 84.0.4147.105 (Official Build) (64bit) — Preceding unsigned comment added by 75.109.252.140 (talk) 17:11, 4 August 2020 (UTC)
Triple tangent/cotangent identities[edit]
Perhaps my edit of 04:43, 23 November wasn't clear. I believe that, if it is acceptable to give these things names, the way I have done it is correct. I don't understand the objection that "changing the cotangent identity to the sinedouble identity (even if accurate) is not [acceptable]". It wouldn't be accurate, and I never meant to imply such a thing. It is not my intention to give the equation about tangents any name other than the tangent identity, the one about cotangents any name other than the cotangent identity, or the one about sines any name at all. I'm not aware that the one about double sines has any name, though if someone comes up with one, that's fine. SamHB (talk) 16:58, 23 November 2013 (UTC)
Proof of Chebyshev formulae[edit]
Just in case it's useful to anyone, the Chebyshev formulae for and are most easily proved as the imaginary and real parts (respectively) of the formula:
where as in Euler's formula. Because this is an exponential function, it has the particularly simple summation function , and
Given this, the formula can be simplified by dividing both sides by to get:
Which can be proved by expanding and then simplifying the righthand side:
71.41.210.146 (talk) 17:06, 28 December 2013 (UTC)
Symmetry[edit]
Forgive me if I'm being an idiot, but in the table that includes the cofunction identities, should the transformations not be described as reflections in and respectively? M.A.Redman (talk) 19:14, 10 April 2014 (UTC)
useful identity[edit]
It would be handy to give a solution to the geodesic eqn 67.198.37.16 (talk) 20:01, 30 September 2015 (UTC)
If the article gets too long, how shall we reorganize or split it?[edit]
By WP:SIZESPLIT and Wikipedia:Article size pages above 60 kb size should be split List of trigonometric identities is about 71 kb so is in need of a split.
Also the list is ab bit hard to follow so lets have a discussion about how we reorganize this page and maybe if and how how we should split it.
My idea is to split it in two, one with let us call it the high school trigonometry and a second one with more advanced subjects.
Advanced subjects would for me include:
 List of trigonometric identities#Historical shorthands maybe better at a page historical trigonometric functions
 List of trigonometric identities#Angle sum and difference identities matrix forms
 Sines and cosines of sums of infinitely many terms
 Tangents of sums
 Secants and cosecants of sums
 List of trigonometric identities#Multipleangle formulae
 Chebyshev polynomial
 Hermite's cotangent identity
and many more lets discuss before we edit that makes nobody happy. WillemienH (talk) 12:44, 25 February 2016 (UTC)
 I think the article is on a coherent single topic and so should not be split up. I don't think it's hard to navigate, since there's a table of contents. With two separate articles, someone might have a hard time finding what they're looking for, if they go to the wrong article. Loraof (talk) 20:31, 5 June 2016 (UTC)
 I agree. I don't think splitting up is a good idea. The cited guidelines with the 60kb are more than a crude rule of thumb than an absolute figure. We have plenty of compilation and overview articles that are larger than 60kb and where splitting them up is simply not a good option. In fact many of our excellent articles are > 70kb.Kmhkmh (talk) 20:53, 5 June 2016 (UTC)
More than two sinusoids[edit]
As in the "Arbitrary phase shift" and "Sine and cosine" cases the expressions for a and θ should be unambiguous. To the best of my knowledge that is
and
— Preceding unsigned comment added by Tpreclik (talk • contribs) 03:39, 3 January 2017 (UTC)
Pythagorean Identity[edit]
When I learned the following identities, they were all called "Pythagorean identities" with consistency from teacher to teacher:
However, online it seems that some people only use the term "Pythagorean identity" to refer to the former. Therefore, would anybody object to me changing "identity" to "identities"? Thank you.LakeKayak (talk) 19:18, 18 February 2017 (UTC)
 As it seems that nobody has an opinion, I am going to make the change and wait for people's responses from there.LakeKayak (talk) 01:28, 21 February 2017 (UTC)
atan2[edit]
I recommend removing all mention of atan2 from the article, as atan2 is a computer programming language function—not a standard trigonometric function.—Anita5192 (talk) 16:35, 10 November 2019 (UTC)
I support that. There are only two sections where atan2 is mentioned:
 There's an identity for atan2 in the table under List_of_trigonometric_identities#Angle_sum_and_difference_identities that is equivalent to the one given for arctan, and is also stated in Atan2#Angle_sum_and_difference_identity. It's not cited here, although a proof is given there.
 The other section is List_of_trigonometric_identities#Linear_combinations. Looking at the sources, the Mathworld one doesn't actually use atan2, the Cazelais source is a broken link, and I can't verify the Apostol source but it's unlikely that it uses atan2. (It's dated 1967, while atan2 was first introduced to Fortran in 1961, and I doubt the terminology moved into a standard calculus text that early.)
So neither of these uses of atan2 are actually supported by the sources, at least that I can verify. The first one can be removed without any fuss. The second one can be rewritten in terms of standard arctan based on the Mathworld article. Apocheir (talk) 23:50, 10 November 2019 (UTC)
Sure, atan2 originates from programming languages, but it is nevertheless a perfectly valid mathematical function. It would be useful for the linear combination section, because
can be made to work for all a and b by using
with the interpretation that
The way it is now, using atan, leaves an unnecessary singularity at a = 0, and the equations do not hold there. The phase range is also only 180 degrees instead of 360, and amplitudes can take on negative values, which is a bit strange. If it's the programming language origin of atan2 that's the main issue, then you can equivalently use the arg function as above.
Perhaps one of the most common places for this linear combination to occur is in the Fourier series, where one converts
into
That article defines
 and
unabashedly using the atan2 function. In this prototypical application it would be strange to use a 180 degree phase range and negative amplitudes, which is part of the reason why such a parametrization feels weird to me. More generally, a full circle is the most natural range for angles, and amplitudes are most natural if they're nonnegative.
I don't unfortunately have suitable sources at hand, so I'm leaving the article asis, but if someone finds such then I suggest using arctan2 in the article. You can also use arg, but there's no need to introduce a complex function into a real context, no matter how suited they are for expressing things related to the unit circle.  StackMoreLayers (talk) 02:13, 12 April 2021 (UTC)
Significant identities[edit]
There are a great many existing trigonometric identities. However, this article cannot contain all of them and has space only for the more significant identities. Please do not insert identities that are not in common use and especially do not include a huge list of them.—Anita5192 (talk) 17:49, 17 November 2019 (UTC)
 I wholeheartedly agree and would go even further. There are many identities on this page that are not sourced or sourced to unreliable sources. The page seems to be a magnet for people who want to see "their" identities in print and needs to be pruned from time to time to weed these things out. I've also noticed some basic identities (that should be in the list) without citations, so a straightforward weeding would have to be done with care and is probably a job for more than one editor.Bill Cherowitzo (talk) 23:08, 17 November 2019 (UTC)