# Incredibly useful math concepts that everyone should know how to use **URL:** \ **Category:** In My Humble Opinion\ **Created:** [July 26, 2024, 7:57pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826 "2024-07-26T19:57:35Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Nars\_Glinley](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nars_glinley/32/8177_2.png) [@Nars\_Glinley](https://boards.straightdope.com/u/Nars_Glinley)\ **Post date:** [July 26, 2024, 7:57pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/1 "2024-07-26T19:57:35Z")
If I had a nickel for every time that I’ve used cross multiplication to figure out an unknown quantity, I would have an extraordinary amount of nickels. I’ve used the Pythagorean theorem on occasion and even a little trig now and then but none of them have been as frequent as cross multiplying. I’m sorry Mr. Jackson, even though you made each of us get up and sing the quadratic formula in Algebra class, I have yet to use it as an adult. I wish I had because I still remember it after 40+ years. Outside of basic arithmetic, what other math concepts should non-math majors be able to incorporate into their not really “daily” lives? Before I hit submit, using pi comes to mind as well. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [July 26, 2024, 8:09pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/2 "2024-07-26T20:09:39Z")
I use the Pythagorean theorem when felling a tree to determine where the top of the tree will end up. In one case my gf thought we should hire a pro. I assured her I was totally good to go. I dropped the tree and my gf was really impressed. The top of the tree was _inches_ away from a wall I wanted to miss. I thought I would be 5 feet away. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 26, 2024, 8:34pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/3 "2024-07-26T20:34:48Z")
I’d expand the mention of cross-multiplication a bit, to say proportions in general. Just the other day, I measured the height of a lamp-post by measuring its shadow, and the height and shadow of a shorter post nearby. ---
**Author:** ![Llama\_Llogophile](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@Llama\_Llogophile](https://boards.straightdope.com/u/Llama_Llogophile)\ **Post date:** [July 26, 2024, 8:43pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/4 "2024-07-26T20:43:48Z")
I’m more of an advocate for basic statistical concepts. Understanding correlation, standard deviation and probability has been more useful to me than most purely mathematical ideas. ---
**Author:** ![sunacres](https://avatars.discourse-cdn.com/v4/letter/s/4491bb/32.png) [@sunacres](https://boards.straightdope.com/u/sunacres)\ **Post date:** [July 26, 2024, 8:45pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/5 "2024-07-26T20:45:16Z")
The sensible use of significant digits. Ubiquitous calculators have made even basic arithmetic nearly optional, but folks have a hard time with what all those digits mean. ---
**Author:** ![Slithy\_Tove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slithy_tove/32/18248_2.png) [@Slithy\_Tove](https://boards.straightdope.com/u/Slithy_Tove)\ **Post date:** [July 26, 2024, 8:59pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/6 "2024-07-26T20:59:16Z")
> [@kayaker](#): > > I use the Pythagorean theorem when felling a tree to determine where the top of the tree will end up. In one case my gf thought we should hire a pro. I assured her I was totally good to go. > > I dropped the tree and my gf was really impressed. The top of the tree was _inches_ away from a wall I wanted to miss. I thought I would be 5 feet away. I wanted to do that once too, looking at a treetop through a paper towel tube pinned to the protractor on a carpenter’s square, making sure the bubble was centered. But I realized I lacked the math skills to account for my eye-level elevation, and knew how stupid I’d look standing eye-deep in a hole looking through a paper towel tube. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [July 26, 2024, 9:11pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/7 "2024-07-26T21:11:44Z")
It’s pretty straightforward. The protractor device would tell you how high above your eye level the tree-top was, and given the imprecision in the whole system, you’d be pretty safe just adding your height to result you got from the protractor. ---
**Author:** ![VOW](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/vow/32/3043_2.png) [@VOW](https://boards.straightdope.com/u/VOW)\ **Post date:** [July 26, 2024, 9:22pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/8 "2024-07-26T21:22:43Z")
Ratios. Anything with fractions. Calculators don’t do feet and inches, unless you convert everything to decimals. But, fractions. Sewing (how many yards of each fabric? How many packages of bias tape?), cooking (divide 3-1/2 cups in half). Things get interesting when the room is 11 feet 8 inches by 9 feet. How many square yards of carpet should you buy? How many rolls of wallpaper? Nevermind. I hate wallpaper. When we were planning to finally move to our property, I showed Mr VOW exactly where I wanted the house. We paid a guy with a backhoe (and whatever the front gizmo was, LOL) to level the house site, and to dig the hole where the engineer would play in the dirt so we could get the septic permit. Mr VOW and I had stakes and string, plus the dimensions of the mobile home, and we figured out exactly where the house would go. I Pythagorized the Hell out of those string lengths and stake placements, even doublechecked with diagonal measurements. Backhoe guy was super-appreciative, said most people don’t go that far. He leveled the land beautifully, even graded it 3% to drain. Yes, Children, do pay attention in your math classes! Your calculator may have functions and programs up the wazoo, but unless you understand the basics, all the calculator can do is eat batteries. ~VOW ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [July 26, 2024, 9:24pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/9 "2024-07-26T21:24:29Z")
As an electrician, basic algebra is a necessity and is used all the time for calculating volts, watts, etc. The Pythagorean theorem is also handy for several applications in general construction, and knowing how to find the volumes of cubes and cylinders is very important. When you’re ordering concrete, you want to be accurate. ---
**Author:** ![Q.Q.Switcheroo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/q.q.switcheroo/32/5713_2.png) [@Q.Q.Switcheroo](https://boards.straightdope.com/u/Q.Q.Switcheroo)\ **Post date:** [July 26, 2024, 9:29pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/10 "2024-07-26T21:29:06Z")
> [@kayaker](#): > > I use the Pythagorean theorem when felling a tree to determine where the top of the tree will end up. It’s also handy for doing Buster Keaton impressions. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [July 26, 2024, 9:37pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/11 "2024-07-26T21:37:17Z")
> [@Q.Q.Switcheroo](#): > > It’s also handy for doing Buster Keaton impression Turns out we both look good in a hat worn at a rakish angle, so there’s that…… ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [July 26, 2024, 11:10pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/12 "2024-07-26T23:10:42Z")
Many people could avoid some simple errors and take advantage of good opportunities if they had a better understanding of probability. I think it would also prevent them ascribing things to forces other than coincidence so often. ---
**Author:** ![Stratocaster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stratocaster/32/16393_2.png) [@Stratocaster](https://boards.straightdope.com/u/Stratocaster)\ **Post date:** [July 26, 2024, 11:43pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/13 "2024-07-26T23:43:27Z")
> [@kayaker](#): > > I dropped the tree and my gf was really impressed. The top of the tree was _inches_ away from a wall I wanted to miss. I thought I would be 5 feet away. Well, I hope you never admitted it. “Yep, exactly as I calculated” would be the proper reaction. ---
**Author:** ![Gatopescado](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gatopescado/32/5711_2.png) [@Gatopescado](https://boards.straightdope.com/u/Gatopescado)\ **Post date:** [July 27, 2024, 12:16am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/14 "2024-07-27T00:16:44Z")
> [@Slithy\_Tove](#): > > …and knew how stupid I’d look standing eye-deep in a hole looking through a paper towel tube. This kills me. 😆 On many levels. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [July 27, 2024, 2:33am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/15 "2024-07-27T02:33:34Z")
I occasionally use the formula to come up with the sum of a series of numbers. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [July 27, 2024, 2:53am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/16 "2024-07-27T02:53:50Z")
> [@kayaker](#): > > I use the Pythagorean theorem when felling a tree to determine where the top of the tree will end up. In one case my gf thought we should hire a pro. I assured her I was totally good to go. > > I dropped the tree and my gf was really impressed. The top of the tree was _inches_ away from a wall I wanted to miss. I thought I would be 5 feet away. There’s more to that than just geometry, though. As the tree starts to fall, it picks up some sideways momentum, and there’s nothing to stop it. In other words, the top of the tree ended up inches away from the wall. The bottom of the tree was probably about 4 feet away from the stump. > [@Andy\_L](#): > > It’s pretty straightforward. The protractor device would tell you how high above your eye level the tree-top was, and given the imprecision in the whole system, you’d be pretty safe just adding your height to result you got from the protractor. I learned how to use a sextant to determine latitude at sea. You use the sextant to find the angle between the horizon and the bottom of the solar disk. Since you’re looking slightly downward at the horizon, you make a correction based on the height of your eye above the water. You also correct for the radius of the sun. My final answer was off by about 20 miles. The math concept I use is mutiplying by 1, but you have to write 1 in the correct way. When I’m converting units, like miles to kilometers, I can never remember if I need to multiply or divide. So I turn it into a fraction, like 16km / 1 mi. If the top and bottom of a fraction are equal, then the fraction equals 1. And you can multiply anything by 1. The units cancel out, and I have the answer. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [July 27, 2024, 3:11am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/17 "2024-07-27T03:11:01Z")
Not really a math concept but more a little trick I’ve taught friends who say “I’m so bad at math” when trying to figure out the tip on a bill. To figure out a straight 20% take the total you want to tip on, move the decimal one place left, then double it. ---
**Author:** ![susan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/susan/32/17537_2.png) [@susan](https://boards.straightdope.com/u/susan)\ **Post date:** [July 27, 2024, 3:27am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/18 "2024-07-27T03:27:04Z")
When I’m walking, I calculate the hypotenuse of my path’s angles for fun. ---
**Author:** ![Roderick\_Femm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/roderick_femm/32/14875_2.png) [@Roderick\_Femm](https://boards.straightdope.com/u/Roderick_Femm)\ **Post date:** [July 27, 2024, 3:42am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/19 "2024-07-27T03:42:30Z")
Percentages. Understanding why not to use a phrase like “200% cheaper,” and how to interpret such lunacy when you come across it. ---
**Author:** ![RayMan](https://avatars.discourse-cdn.com/v4/letter/r/e495f1/32.png) [@RayMan](https://boards.straightdope.com/u/RayMan)\ **Post date:** [July 27, 2024, 6:03am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/20 "2024-07-27T06:03:15Z")
> [@Robot\_Arm](#): > > The math concept I use is mutiplying by 1, but you have to write 1 in the correct way. When I’m converting units, like miles to kilometers, I can never remember if I need to multiply or divide. So I turn it into a fraction, like **16km / 1 mi**. If the top and bottom of a fraction are equal, then the fraction equals 1. And you can multiply anything by 1. The units cancel out, and I have the answer. One mile is approximately 1.6 kilometers. [Next page](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826.md?page=2)