# 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:** 18\ **Page:** 7
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 6, 2024, 11:51am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/122 "2024-08-06T11:51:32Z")
> [@kayaker](#): > > > [@Hari\_Seldon](#): > > > > I still don’t fully understand the Riemann-Roch theorem. > > That makes two of us. At least. On the other hand, I met somebody who told that he got into geometry precisely because he found it easy to visualize. So YMMV. ---
**Author:** ![sunacres](https://avatars.discourse-cdn.com/v4/letter/s/4491bb/32.png) [@sunacres](https://boards.straightdope.com/u/sunacres)\ **Post date:** [August 6, 2024, 9:09pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/123 "2024-08-06T21:09:24Z")
> [@aceplace57](#): > > I remember older generations had intuitive math skills. This statement is breathtaking to me, especially on a board ostensibly dedicated to well-supported and rational discourse. Maybe you meant “I knew some folks from older generations who had intuitive math skills”? I take it back, we’re in IMHO. Nevermind. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 7, 2024, 12:08am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/124 "2024-08-07T00:08:10Z")
I do, of course. When I was more active in research, I got some good ideas while trying to fall asleep or walking to my office. I still think about problems I solved long ago, thinking about the solutions. ---
**Author:** ![Al128](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/al128/32/16541_2.png) [@Al128](https://boards.straightdope.com/u/Al128)\ **Post date:** [August 7, 2024, 7:01pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/125 "2024-08-07T19:01:16Z")
overall, I am pretty good at math … (donno prob. top 5-10% of the overall population) … but I cannot for the life of me make divisions on paper (like I learned 45+ years ago) … stuff like **89,642 / 712 = ???** (and by that I mean I do not recall the algorithm to do it) but I am pretty good at getting a “close number” of that in my head w/out pen/paper … (I just did the basic 10 sec test and came to “120 to 130” as a result … (my cel. tells me the result is 125.9) good enough for 95% of all ocurrances in my life - for the remaining 6 % I have a celphone 😉 ---
**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:** [August 7, 2024, 7:21pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/126 "2024-08-07T19:21:00Z")
> [@Al128](#): > > but I cannot for the life of me make divisions on paper (like I learned 45+ years ago) Very few people do. You learn that in about third grade, and then your teachers say “And now that we’ve shown that we know how to do that, we can use a calculator”. And then students get to Algebra II and learn how to divide polynomials, which uses the same process but for which there isn’t a button on a calculator, and everyone goes “Wait, I used to know how to do that”. And yes, it is the same process (if anything, a touch simpler, because you don’t have to worry about borrowing). This is because place-value numbers are polynomials. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 7, 2024, 7:25pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/127 "2024-08-07T19:25:53Z")
> [@Al128](#): > > but I cannot for the life of me make divisions on paper (like I learned 45+ years ago) The point is that with mathematical skills/concepts you can figure it out. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [August 7, 2024, 10:42pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/128 "2024-08-07T22:42:27Z")
> [@Al128](#): > > stuff like **89,642 / 712 = ???** > (and by that I mean I do not recall the algorithm to do it) > [@Chronos](#): > > Very few people do. You learn that in about third grade, and then your teachers say “And now that we’ve shown that we know how to do that, we can use a calculator”. Do kids still learn this? I did, but I can’t remember the last time I’ve actually done it. Still, I solved it without too much difficulty (125 Remainder 642 — the numbers you chose worked out fairly easily), but I wouldn’t expect the average person to do so. ---
**Author:** ![Thing.Fish](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thing.fish/32/9168_2.png) [@Thing.Fish](https://boards.straightdope.com/u/Thing.Fish)\ **Post date:** [August 8, 2024, 3:52am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/129 "2024-08-08T03:52:33Z")
P & E would be a lot less frustrating if people in general understood probability. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 8, 2024, 10:31am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/130 "2024-08-08T10:31:41Z")
> [@Al128](#): > > overall, I am pretty good at math … (donno prob. top 5-10% of the overall population) … > > but I cannot for the life of me make divisions on paper (like I learned 45+ years ago) … > > stuff like **89,642 / 712 = ???** > (and by that I mean I do not recall the algorithm to do it) > > but I am pretty good at getting a “close number” of that in my head w/out pen/paper … (I just did the basic 10 sec test and came to “120 to 130” as a result … (my cel. tells me the result is 125.9) > > good enough for 95% of all ocurrances in my life - for the remaining 6 % I have a celphone 😉 Having been shown how to do it by hand is useful for developing a conceptual understanding that helps you quickly grasp the order of magnitude of a calculation even before reaching for an actual calculator. Trigonometry classes in high school or college do something similar for things like the sine and cosine of an angle. Several decades ago when slide rules were still in common use, the people who used them regularly got good at estimating results because it was helpful when using a slide rule. A slide rule works by turning a difficult multiplication or division operation into an easy addition or subtraction problem. To do long division, you can take the logarithm (log) of each number, subtract one from the other, and take the inverse log of the resulting number to get your final answer. This is great, because subtraction is much easier to do by hand than long division is. A slide rule is a really handy tool that gives you the logarithm of a number, but the catch is that it can only do this for numbers between 0 and 10. So you move the decimal points on your starting numbers: 89,642 → 8.9642 712 → 7.12 The log of 8.9642 is 0.95251. The log of 7.12 is 0.85248. The difference of these two is 0.10003154. The inverse log of 0.10003154 is 1.259. To get from 1.259 to the final answer, you need to remember the total number of spaces you moved the decimal points in your starting number (four for the first number, minus two for the second number (_minus_ because you’re doing division instead of multiplication)). Or you can skip that, and just round your starting numbers to the nearest decade (10, 100, 1000, etc.): 89,642 → 100,000 712 → 1,000 Now divide 100,000 by 1,000, which is really easy to do in your head: the answer is 100. Now you know your final answer should be near 100, so you take your slide-rule answer of 1.259, and move the decimal to make your answer near 100: 1.259 → 125.9 Easier than trying to keep track of decimal point movements earlier in the operation. Being able to round numbers so that you can quickly calculate an estimate in your head is handy - and if you can develop a sense of how big your rounding errors are on your inputs, you can hedge your estimate, which is even more handy. In this case, I can tell you that the denominator (712) moved relatively farther to round up to 1000 than the numerator (89,642) moved to round up to 100,000, so I know my answer will definitely be somewhat greater than 100, definitely not less than 100. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 11, 2024, 12:07am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/131 "2024-08-11T00:07:09Z")
> [@DPRK](#): > > > [@Al128](#): > > > > but I cannot for the life of me make divisions on paper (like I learned 45+ years ago) > > The point is that with mathematical skills/concepts you can figure it out. Here is an example: remember that algorithm you may or may not have learned to take square roots via long division? Nobody does, because it’s not that useful. However, given a bit of time you can reconstruct it, assuming you more or less understand how it is supposed to work. ---
**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:** [August 11, 2024, 1:04am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/132 "2024-08-11T01:04:37Z")
I remember it - it’s fun to play with during boring meetings ---
**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:** [August 12, 2024, 2:26am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/133 "2024-08-12T02:26:21Z")
> [@Machine\_Elf](#): > > Several decades ago when slide rules were still in common use, the people who used them regularly got good at estimating results because it was helpful when using a slide rule. I still teach a slide rule lesson to my high schoolers, whenever we do logarithms (algebra 2 and pre-calc), just because it’s so good for illustrating concepts like that. > [@DPRK](#): > > Here is an example: remember that algorithm you may or may not have learned to take square roots via long division? I never learned that, but I have whiled away time by coming up with algorithms for square roots. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 12, 2024, 10:56pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/134 "2024-08-12T22:56:14Z")
I more or less remember it–or can reproduce it. But here is an interesting story about it. A number of years ago (probably nearly 40) I bought a Forth (an old computer language) interpreter. It came with a built-in square root program that worked by taking a trial square root, say t (I think they started with t=1, and averaging t with n/t where n is the original number. Note that if t is the square root, then t = n/t. Rinse and repeat until you get t = n/t (or nearly so). It was very slow since on the 8088 chip division is very slow and each iteration used 2 divisions, one to get n/t and the other to do the average (although an obvious optimization is shift right to divide by 2, so maybe it used only one). But I recalled that seventh grade algorithm. It was basically choose a trial next digit and, if it turns out to be too large reduce it. Well, in binary, the next bit can be only 0 or 1. Always try 1 and if it is turns out to be too large, choose 0 and go to the next stage. It was easy to program and ran rings around the original algorithm. Each step used only a subtraction and a comparison. I sent it off the vendor and I think he used it in his next iteration. ---
**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:** [August 13, 2024, 12:19am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/135 "2024-08-13T00:19:49Z")
> [@Hari\_Seldon](#): > > A number of years ago (probably nearly 40) I bought a Forth (an old computer language) interpreter. It came with a built-in square root program that worked by taking a trial square root, say t (I think they started with t=1, and averaging t with n/t where n is the original number. Note that if t is the square root, then t = n/t. Rinse and repeat until you get t = n/t (or nearly so). You can improve that algorithm, to only need one division, total (no matter how many iterations you do). First off, keep all of your approximations as rational numbers, numerator over denominator, so you can invert a number just by swapping the two. Second, all the algorithm needs is that you pick a new number between t and n/t, not that it be any particular average, and a quick-and-dirty way to do that with rational numbers is to add both the numerators and denominators. For instance, for sqrt(2). Start with 1, or 1/1. 2 divided by that is 2/1. So our next approximation is \frac{1+2}{1+1}, or \frac{3}{2}. After that, we want a number between \frac{3}{2} and \frac{4}{3}. One possibility is \frac{3+4}{2+3}, or \frac{7}{5}. Then the next approximation is \frac{17}{12}, then \frac{41}{29}, etc. Once you’ve taken as many iterations as you want, only then do you actually do the division (for instance, \frac{41}{29} \approxeq 1.4138, compared to \sqrt{2}\approxeq 1.4142). ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 13, 2024, 1:19am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/136 "2024-08-13T01:19:58Z")
Yes that works, but that is not what the given program did. That would have been a vast improvement, but my point is that the 7th grade algorithm becomes really easy in binary. ---
**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:** [August 13, 2024, 1:55am UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/137 "2024-08-13T01:55:19Z")
> [@Hari\_Seldon](#): > > A number of years ago (probably nearly 40) I bought a Forth (an old computer language) interpreter. It came with a built-in square root program that worked by taking a trial square root, say t (I think they started with t=1, and averaging t with n/t where n is the original number. Note that if t is the square root, then t = n/t. Rinse and repeat until you get t = n/t (or nearly so). In the early 80s I read an article in Popular Science about a similar algorithm for calculating roots of any positive integer degree. Divide x by t to the (i-1) power, add (i-1) times t, and divide by i. So for x=8 and an initial cube root guess of 1, you get (8+2)/3 as the next guess, and then 2.462, then 2.081 and so on to 2 ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 13, 2024, 8:17pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/138 "2024-08-13T20:17:11Z")
> [@Chronos](#): > > or instance, for sqrt(2). Start with 1, or 1/1. 2 divided by that is 2/1. So our next approximation is \frac{1+2}{1+1}1+21+1\frac{1+2}{1+1}, or \frac{3}{2}32\frac{3}{2}. After that, we want a number between \frac{3}{2}32\frac{3}{2} and \frac{4}{3}43\frac{4}{3}. One possibility is \frac{3+4}{2+3}3+42+3\frac{3+4}{2+3}, or \frac{7}{5}75\frac{7}{5}. Then the next approximation is \frac{17}{12}1712\frac{17}{12}, then \frac{41}{29}4129\frac{41}{29}, etc. Once you’ve taken as many iterations as you want, only then do you actually do the division (for instance, \frac{41}{29} \approxeq 1.41384129≊1.4138\frac{41}{29} \approxeq 1.4138, compared to \sqrt{2}\approxeq 1.4142√2≊1.4142\sqrt{2}\approxeq 1.4142). At first, I thought this would work. But on thinking about it I realized that you had to know whether the latest number was above the square root in question or below in order to know which two of the last three to take the Farey average (as it is known) of. This works well with the square root of 2, but consider, say, 5. The first two numbers will be 1/1 and 5/1 and the third is 6/2. Now we can see that (6/2)^2\>5, but if we want to design an automated algorithm to do it, we have to whether 6^2 is less than or greater than 2^2\times 5 and this requires 3 multiplications which is also slow at least on the 8088. If you ignore this your sequence will go 1/1,5/1,6/2,11/3,17/5,28/8,45/13,73/21,… which seems to be heading towards about 3.5. These are actually Fibonacci type sequences and it would not be hard to calculate, but not worth the effort. ---
**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:** [August 13, 2024, 11:17pm UTC](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826/139 "2024-08-13T23:17:54Z")
> [@Hari\_Seldon](#): > > But on thinking about it I realized that you had to know whether the latest number was above the square root in question or below in order to know which two of the last three to take the Farey average (as it is known) of. You’re not picking two of the last three. You’re picking two of the last two. At every stage i, you have an ai and a bi, both of which are approximations of sqrt(x). For simplicity, we can take a0 = 1 and b0 = x, if you like (though in practice, you can usually find a better starting approximation). Then, ai+1 is the Farey average of ai and bi, and bi+1 = x/ai+1 . At any given step, we don’t necessarily know which of a or b is larger, but we know that one of them is greater than the true square root and one is less, and that any number in between them is therefore a better approximation. Using your example of 5: ```auto n a_i b_i a (decimal) 0 1 5 1 1 6/2 10/6 3 2 16/8 40/16 2 3 56/24 120/56 2.33333 4 176/80 400/176 2.2 5 576/256 1280/576 2.25 6 1856/832 4160/1856 2.23077 ... inf 2.23607 ``` [Previous page](https://boards.straightdope.com/t/incredibly-useful-math-concepts-that-everyone-should-know-how-to-use/1004826.md?page=6)