# High School Math in the Talmud

I'm currently tutoring someone in math that has an atitude of "what's the point of all this... why would I ever need this..." (which is typical of many high-schoolers). To try to pique his interest, I figured it would be great if I could provide him with examples from the Gemara that actually make use of some of the regent-level math he's learning.

So my question is what are some examples from the Talmud (Rashi and Tosfos included) that make use of (or refer to) high school level math?

• How about ways one would use math practically for Halachic purposes? – Isaac Moses Dec 19 '12 at 3:52
• I recommend you look through the archives of cheshbon.weeklyshtikle.com – Isaac Moses Dec 19 '12 at 3:54
• If that won't impress him, you can tell him about this guy you know through an internet forum who frequently has to divide fractions at work for reasons that are difficult to explain in a comment box. :) – Seth J Dec 19 '12 at 4:08
• Really, if a person doesn't find math intrinsically beautiful, it's probably hard to make a specific case for learning past arithmetic, given that most math is abstracted away from us in 21st c. Western society. Might want to look at the remarkable manifestations of math constructs in nature. Basic algebra is certainly related to logic though, so a good foundation in that is a good thing WRT Talmud. He should be able to cross over from e.g. kal va-chomer and represent that quasi-algebraically (if a > b etc.) – yitznewton Dec 19 '12 at 5:50
• – msh210 Apr 2 '14 at 1:42

Try Pesachim 109a-b where the Gemara (and more elaborately in Rashi and Tosfot) tries to work out the volume of a Reviit in Etzba^3 based on its knowledge of the volume of a Mikva in Amah^3 (ie lots of basic algebra and unit conversion).

• It gets pretty complicated, so be warned. – Double AA Dec 19 '12 at 3:24
• Good stuff. I find the hardest part about it to be the terminology, while in terms of math concepts it is right up there at high school level. – WAF Dec 19 '12 at 4:45
• @WAF Makes you glad we have a decimal system! – Double AA Dec 19 '12 at 5:00
• Yeah it's actually not that bad to take a calculator and verify the Gemara's result of "how many cubic fingers make a quarter-log of volume?" The problem is tracking the way various medieval commentaries do the math -- they use some very convoluted methods to avoid fractions until the bitter end! (And then Tosfos says the Yerushalmi has a different number, it meant a round cup instead of a cubed one ...) – Shalom Dec 19 '12 at 8:46

I remember doing the gemara on Sukkah 8a in high school while I was also in a geometry class in the afternoons. It's pretty basic high-school geometry stuff. Squares and circles. It's the Tosfos there, though, that go all out.

It's particularly ingenious how Tosfos (bottom of the page) demonstrates that the ratio of the diagonal of a square to its side (which we know is sqrt(2)) is not exactly 7/5 as the gemara asserts.

In Hakirah vol. 14, they published an article called "'Learning' Mathematics" which includes examples of different mathematical applications in classical Jewish literature.

You might want to show him Rambam Hil. Kiddush HaChodesh and the diagrams in the back, as well as Chazon Ish on Kiddush HaChodesh and the attendant illustrations. At the very end the Chazon Ish even includes a handy sine table! In R' Chaim Kanievsky's Shekel HaKodesh there is an appendix that explains the trigonometric underpinnings of the numbers given by the Rambam.

The Mirkeves HaMishneh wrote a kuntres called Breichos B'Cheshbon (included in some editions of Mirkeves HaMishneh) which provides advanced mathematical explanations for various sugyos. About 30 years ago it was translated into English and explained, in a book called Approaching Infinity. It was a fascinating read and is linked as a PDF.

• Approaching Infinity seems to be on re-order from this site. I guess you could get it there later (or on Barnes & Noble now, but I doubt you want to pay \$73) – b a Dec 19 '12 at 4:37
• @IsaacMoses If anyone is looking for a copy of Di Shemaya, I still have a few boxes stored in my garage. (Alex asked me to be his Lakewood "distribution" point back when he published.) – LazerA Dec 19 '12 at 6:48
• I have a pdf of Approaching Infinity from a scan I made since the book is not available for purchase. If anyone wants a copy ping me. – Yoni Apr 14 '16 at 13:31
• @Yoni - That's fantastic! Would you be able to upload it and supply a link? – Dave Apr 15 '16 at 16:03
• @Yoni Can you please supply a link for the Approaching Infinity Pdf? Thanks! – Yaakov Pinchas Mar 29 at 15:15

The gemara (Y'vamos 82b) discusses iteratively replacing mikva water with fruit juice.1 The case involves adding one se'ah of fruit juice to a 40 se'ah mikva, and then removing one se'ah of the solution. R' Yochanan rules that this iterative process may be repeated so long as more than 50 percent (or at least 50 percent) of the solution remains water.

Rashi (s.v. mai lav) seems to comment that no more than 19 iterations may be performed.2 Tosafos Y'shanim (ad loc.) points out that 20 iterations does not mathematically get you below 50 percent (assuming some mixing of the solution occurs during this process), but concludes that 20 iterations would still render the mikva rabbinically invalid because it "appears like a majority" of the solution is fruit juice.

Rashi's wording ("d'lo nishkol ruba, aval ad palga shapir dami") does not strike me as amenable to this approach. My guess is that Rashi limited the iterations to 19 to account for a worst case scenario of virtually no mixing,3 in which case more than 19 iterations would still involve at least a safeik that the mikva is biblically valid.

However, if we could assume that the solution becomes perfectly mixed after each addition of fruit juice, we could use high school math to determine the maximum number of iterations after which the mikva would still remain biblically valid:

Suppose we want strictly more than 20 se'ah of water in the mikva solution. Denote the maximum number of allowable iterations n (∈ R). We can set up the inequality 40*(40/41)^n > 20 ⇒ (40/41)^n > .5 ⇒ log(base 40/41) of .5 < n ⇒ 1/(log(base 2) of 41 - log(base 2) of 40) < n. In this case n is slightly more than 28, so we can denote the integer-valued number of iterations as n' = sup{q ∈ Z | q ≤ n}, i.e. the largest integer less than or equal to n, namely 28.4

(Although I don't think logarithms were in use during Rashi's time, a close enough approximation for n could have been computed numerically in at most a matter of hours).

The halachic ramifications of this perfect mixing scenario are questionable, but I think this case could be used as a construct for a math problem.

1 Or temed, as the case may be. There are different interpretations as to what temed is, one being that it is a form of dilute wine made by soaking the sediment particulates from old wine in water. See Tosafos (s.v. nasan se'ah) for how this could affect the halacha. For the purposes of this answer, fruit juice is assumed to be the substance in question.

2 If Rashi meant that the solution must contain less that 20 se'ah of juice, he presumably would not have used the phrasing "he may do this until 19 se'ah," which implies an integer valued restriction.

3 Such as where 19 se'ah of fruit juice are added to one side of the mikva and 19 se'ah of solution are removed from the other side of the mikva in almost instantaneous succession. Although this seems to be an unfeasible feat, the limit would presumably be set at the edge of the possible rather than the feasible.

4 Please let me know if I made a math error. Incidentally, I wish Mi Yodeya had LaTeX support at times like this.

• Also take a look at Breichos B'Cheshbon, or the out-of-print English adaption Approaching Infinity, which goes through these same calculations. – DonielF Aug 24 '16 at 4:33
• @DonielF Interesting. I notice that sefer is mentioned in this answer, too. – Fred Aug 24 '16 at 5:43

A couple examples from Eruvin come to mind:

Eruvin 14a brings proof that for halachic matters, pi = 3, and then continues to explain the dimensions of Solomon's pool. This contains some prealgebra and simple geometry.

Eruvin 23b discusses the difference between 7.666 and sqrt(5000), with Rashi (s.v. "אלא אמרה תורה טול חמשים") giving an interesting way of calculating the square root of 5000.

You may be interested in my Sefer which includes the Sugyos Eruvim Daf 14,57,76, Pesachim 109, Succos 7,8. It explains Gemara, Rashi, Tosfos, Marsha, Maharom, Gra, others, and presents every step in equation form and diagrams; it also makes corrections and clarifications on diagrams found in the Shas. It includes an Appendix on Basic Algrebra, 21 definitions of Symbols, 32 Laws. Eruvim has 25 equations, Pesachim 106 equations, and Succos 184 equations. This sefer is a small part of what I am preparing to publish Bezras Hashem. In that larger sefer I have treated many other math discussions in Shas and Mishnyos. I am also preparing to present an online course on this topic.

• The links on that page are broken - the ".org" suffix needs to be changed to ".net". – Dave Apr 15 '16 at 16:08
• The links on your site were not working. – Aaron Ross Dec 29 '17 at 18:23

I just discovered this Youtube channel The Math Rebbe. He takes sugyas from different gemaras and commentaries and explains how they use modern high-school and beyond math. I've only just started watching, but it seems interesting. There's currently one (10 episode) season.

The first episode is actually about what i posted here previously.

But beware, he makes some pretty bad puns.

• How did I miss this answer until now? :) – DonielF Nov 22 '17 at 20:01