# What is your favorite equation? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [November 21, 2006, 1:48pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280 "2006-11-21T13:48:49Z")\ **Posts on this page:** 17\ **Page:** 3
**Author:** ![Subway\_Prophet](https://avatars.discourse-cdn.com/v4/letter/s/ecb155/32.png) [@Subway\_Prophet](https://boards.straightdope.com/u/Subway_Prophet)\ **Post date:** [November 22, 2006, 9:57pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/41 "2006-11-22T21:57:46Z")
[QUOTE=Stranger On A Train] Damnit, you beat me to it. Yeah, Euler’s Identity is pure beauty in the form of mathematics, and gives you all the numbers you need to do any mathematical operation (provided you’re prepared to deal with transforms from binary to natural number base arithmetic). And more generally, [Euler’s formula](http://en.wikipedia.org/wiki/Euler%27s_formula) underpins all of plane trigonometry, and allows us to cope in linear fashion with the otherwise nonlinear phenomena of circular and periodic motion via complex analysis. [/QUOTE] **Stranger** , thank you, thank you, thank you. Having read that Wikipedia article on Euler’s formula, I think I finally GET IT. :eek: e[sup]pi i[/sup] = -1 + 0i The 1 and the 0 in Euler’s identity make for a pretty poster, but the real significance of the constant **1** (or -1) is that it’s a coordinate on the complex plane. I swear I’ve tried to work this out in my head from a half dozen textbooks and Mathworld, but it never clicked until I saw [that picture](http://en.wikipedia.org/wiki/Image:Euler%27s_formula.svg). I doubt I would’ve seen that article had you not linked to it! I’m totally geeking out over this. I’m calling my Dad, he’s been trying to explain it to me for decades. ---
**Author:** ![Giraffe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giraffe/32/129_2.png) [@Giraffe](https://boards.straightdope.com/u/Giraffe)\ **Post date:** [November 22, 2006, 10:03pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/42 "2006-11-22T22:03:31Z")
Definitely Schrödinger’s Equation. Simple, yet impossible to solve for all but the simplest cases. What’s not to like? ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [November 22, 2006, 10:09pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/43 "2006-11-22T22:09:56Z")
[QUOTE=Giraffe] What’s not to like? [/QUOTE] How some smart alec can get an umlaut, and probably spell it correctly too? 🙂 ---
**Author:** ![Giraffe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giraffe/32/129_2.png) [@Giraffe](https://boards.straightdope.com/u/Giraffe)\ **Post date:** [November 22, 2006, 10:14pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/44 "2006-11-22T22:14:57Z")
[QUOTE=carnivorousplant] How some smart alec can get an umlaut, and probably spell it correctly too? [/QUOTE] If it makes you feel better, I don’t actually remember the code for ö off the top of my head. I just did a Google search and cut and pasted. 😛 ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [November 22, 2006, 10:20pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/45 "2006-11-22T22:20:38Z")
[QUOTE=Giraffe] I don’t actually remember the code for ö off the top of my head. [/QUOTE] I think you use trigonometric substitution and take the third order partial derivative with respect to ASCII. ---
**Author:** ![Giraffe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giraffe/32/129_2.png) [@Giraffe](https://boards.straightdope.com/u/Giraffe)\ **Post date:** [November 22, 2006, 10:23pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/46 "2006-11-22T22:23:44Z")
[QUOTE=carnivorousplant] I think you use trigonometric substitution and take the third order partial derivative with respect to ASCII. [/QUOTE] No, that would be ß. [sub]I’m starting to see why you dropped that physics class, though…[/sub] ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [November 23, 2006, 12:07am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/47 "2006-11-23T00:07:13Z")
[QUOTE=Giraffe] No, that would be ß. [/QUOTE] You forget that d sin(x) dx is -cos(x). Or is it the other way around? _Dammit._ ---
**Author:** ![Captain\_Carrot](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@Captain\_Carrot](https://boards.straightdope.com/u/Captain_Carrot)\ **Post date:** [November 23, 2006, 12:30am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/48 "2006-11-23T00:30:59Z")
> [@carnivorousplant](#): > > Or is it the other way around? Dammit. Yup, it’s the other way around: d/dx sin(x)=cos(x). ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [November 23, 2006, 12:58am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/49 "2006-11-23T00:58:23Z")
[QUOTE=Squink] it’s hard to beat the tiny rhyme of PV = nRT. [/QUOTE] Also my favorite, I think. It just pops up everywhere. I suppose I should be touting [Manning’s Equation](http://www.engineeringtoolbox.com/mannings-formula-gravity-flow-d_800.html), since it’s so intertwined with how I make my living, but PV=nRT is just so right, so simple and to the point. (Note that this is the third vote for PV=nRT in this thread 😉 ) ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [November 23, 2006, 1:45am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/50 "2006-11-23T01:45:47Z")
[QUOTE=NinetyWt] [Manning’s Equation](http://www.engineeringtoolbox.com/mannings-formula-gravity-flow-d_800.html) [/QUOTE] [OT] I ran across that [Engineering Toolbox](http://www.engineeringtoolbox.com/) website a few months back while trying to figure out [how many watts I needed](http://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html) to maintain the temp of a forced air humidifier. It kicks some serious butt! [/OT] ---
**Author:** ![outlierrn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/outlierrn/32/3005_2.png) [@outlierrn](https://boards.straightdope.com/u/outlierrn)\ **Post date:** [November 23, 2006, 2:00am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/51 "2006-11-23T02:00:10Z")
IANA mathgeek, but I use this equation every day: Dd/Dh x Vh Dd= dose desired Dh= dose on hand Vh= volme on hand simple in concept I confess, but I can I can take an order for xmcg/kg per minute and tell an IV pump xml/hr ---
**Author:** ![Kilvert\_s\_Pagan](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@Kilvert\_s\_Pagan](https://boards.straightdope.com/u/Kilvert_s_Pagan)\ **Post date:** [November 23, 2006, 2:25am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/52 "2006-11-23T02:25:00Z")
The one that is the punch line to the bad joke… Why do (old) computer programmers confuse Halloween and Christmas? Because OCT 31 = DEC 25 Well, this SEEMED like the place to post this… ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [November 23, 2006, 2:36am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/53 "2006-11-23T02:36:09Z")
[QUOTE=Subway Prophet] **Stranger** , thank you, thank you, thank you. Having read that Wikipedia article on Euler’s formula, I think I finally GET IT. :eek: e[sup]pi i[/sup] = -1 + 0i…I doubt I would’ve seen that article had you not linked to it! I’m totally geeking out over this. I’m calling my Dad, he’s been trying to explain it to me for decades. [/QUOTE] Hey, no problem. It’s all a part of the service. I wish I could claim to have written the article, too, but no such luck. This first made genuine sense to me when I was taking Machine Dynamics I. We were having to do all of these 2D vector problems and the author of the text recommended using polar vector notation. I’d seen Euler’s identity before and understood the basic relationship to trigonometry, and I’d dealt with natural exponents in Differential Equations and Linear Systems, and polar notation in my EE classes, but it just seemed to be kind of a historical thing rather than a method of practical use in mechanics. Then I realized I could express planar motion in one equation with natural exponents instead of mucking about with separate equations for the x and y components, and bam! I realized how effing incredible this was. It made complex problems almost trivial to solve. And if you think I’m bad about this, don’t even get me started about [quarternions](http://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation). Unfortunately, outside of computer vector graphics and some esoteric areas of physics, the notation is rarely used or accepted. Still, very useful with time dependent 3D vectors. Stranger ---
**Author:** ![Beware\_of\_Doug](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@Beware\_of\_Doug](https://boards.straightdope.com/u/Beware_of_Doug)\ **Post date:** [November 23, 2006, 5:19am UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/54 "2006-11-23T05:19:53Z")
[QUOTE=Struan] Euler’s formula e^(pi\*i)= -1. [/QUOTE] 'e’s pie-eyed, 'e is! :dubious: ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [November 27, 2006, 8:20pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/55 "2006-11-27T20:20:02Z")
This thread is making me hot. ---
**Author:** ![Malienation](https://avatars.discourse-cdn.com/v4/letter/m/4af34b/32.png) [@Malienation](https://boards.straightdope.com/u/Malienation)\ **Post date:** [December 21, 2006, 8:52pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/56 "2006-12-21T20:52:39Z")
Note to Euler’s fans: just started reading “Dr. Euler’s Fabulous Formula,” by Paul J. Nahin. He shares your sentiment. Upside to book: passionate and lively, unlike many books on the subject of math theory (lotsa actual math in this book, not just an historical account by any means). Downside to book: author’s schtick as snobby, elitist, overeducated jerkoff gets kinda old. ---
**Author:** ![Trunk](https://avatars.discourse-cdn.com/v4/letter/t/87869e/32.png) [@Trunk](https://boards.straightdope.com/u/Trunk)\ **Post date:** [December 22, 2006, 2:03pm UTC](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280/57 "2006-12-22T14:03:34Z")
[QUOTE=gigi] This thread is making me hot. [/QUOTE] 😛 In the spirit of **ultrafilter** ’s post, I like that 6 = 1+2+3 (the sum of its factors) and 28 = 1+2+4+7+14 (the sum of its factors) Also, a sort of mathematical “eye opener” for me was the day when I saw an algebraic representation of the golden ratio. As a young 'un, I’d learned about the spirals, and the squares, and all that crap. Then one day, a high school teacher broke it down for me and reduced finding the ratio to solving x[sup]2[/sup] - x - 1 = 0 It was sort of when I realized the power of mathematics to represent things in the world. [Previous page](https://boards.straightdope.com/t/what-is-your-favorite-equation/381280.md?page=2)