5 = 3 + 2

7 = 5 + 2

12 = 7 + 5

17 = 12 + 5

and so on. Specifically each number is the sum of the immediately preceding one and one of the two numbers summed to make the immediately preceding one.

If you vary the generator by a small amount (and it’s still irrational) you get the same sequence to a point, and then it differs: for a musically significant example, a quarter comma tempered fifth, cents, the sequence is 2, 3, 5, 7, 12, 19… . Notice that the two numbers summed to make 12 are 7 and 5, and the two numbers summed to make 17 are 12, the immediately preceding one, and 5 which is one of the numbers in the sum for 12, while the two numbers summed to make 19 are 12 and 7, the *other* number in the sum for 12. Of course the sequence terminates if the generator is rational: for a generator of 700 cents, for example, it goes 2, 3, 5, 7; then the 8- through 11-note scales are all two-gap, and the 12-note scale is one-gap, i.e., an equal division of the octave. It stops there because further applications of the generator just give back notes you already have.

A generator for a 17-note equal division is cents and a generator for a 19-note equal division is cents. Generators between these two values (other than 700 cents) will give 2, 3, 5, 7, and 12 note two-gap scales, with ones below 700 going on to 19 and ones above 700 going on to 17. Generators a little outside that range will not give 12-note scales; they’ll go 2, 3, 5, 7, 9… if below the range or 2, 3, 5, 8, 13… if above. A diagram from which one can read off the 2-gap scales for generators from 685 to 720 cents is here, and for all generators from 600 to 1200 cents is here (the diagram for 0 to 600 cents is just a mirror image).

Stare at the first of those diagrams and you see 47 appears in two places. That is, the 47-note equal division can be generated by two different generators in that range: cents and cents.

In fact, looking at the whole range from 0 to 1200 cents, there must be 46 generators for a 47-note equal division: for . That’s because 47 is prime. For a 46-note equal division, though, will generate the scale only for odd ; even values will generate only a 23-note scale. And in general, the generators for an *m*-note equal division are for values of where and *n* and *m* are relatively prime. The familiar 12-note equal division has only four generators: , , , and cents.

Examine that diagram some more and you can see how it relates to a sequence of numbers developed as follows: Start with

5, 7

and interpolate the sum of the adjacent numbers

5, 12, 7

(in the diagram, 12 is linked to the horizontal lines associated with 5 and 7); and again

5, 17, 12, 19, 7

(17 is linked to 5 and 12, 19 to 12 and 7); and again

5, 22, 17, 29, 12, 31, 19, 26, 7

(22 is linked to 5 and 17, and so on); and again

5, 27, 22, 39, 17, 46, 29, 41, 12, 43, 31, 50, 19, 45, 26, 33, 7

ad infinitum. Those two values of 47 arise in later iterations of the this sequence from 27+5+5+5 and 33+7+7.

Likewise the big diagram and its mirror image relate to:

1, 1

1, 2, 1

1, 3, 2, 3, 1

1, 4, 3, 5, 2, 5, 3, 4, 1

1, 5, 4, 7, 3, 8, 5, 7, 2, 7, 5, 8, 3, 7, 4, 5, 1

et cetera. We can concatenate the rows of this, dropping the 1s from one end, into a single sequence:

1, 1, 2, 1, 3, 2, 3, 1, 4, 3, 5, 2, 5, 3, 4, 1, 5, 4, 7, 3, 8, 5, 7, 2, 7, 5, 8, 3, 7, 4, 5, …

which we can define with the recurrence relation:

This is known as Stern’s diatomic series, A002487 in OEIS.

From that definition it’s not at all obvious that, for instance, 47 will arise as a sum 46 times while 12 will arise as a sum only 4 times. It’s true though. In fact, consider the following sequence of rational numbers:

1/1, 1/2, 2/1, 1/3, 3/2, 2/3, 3/1, 1/4, 4/3, 3/5, 5/2, 2/5, 5/3, 3/4, 4/1…

That’s just each entry in Stern’s series divided by the subsequent entry. But there’s a theorem: *The nth rational number, in reduced form, can be taken to be b(n)/b(n + 1), for n = 0, 1, 2 … That is, b(n) and b(n + 1) are relatively prime, and each positive reduced rational number occurs once and only once in the list b(0)/b(1), b(1)/b(2), … *Or to put it another way, Stern’s series provides a way to enumerate the rational numbers.

The theorem’s proved in Calkin, N., & Wilf, H. (2000). Recounting the Rationals. *The American Mathematical Monthly, 107*(4), 360-363. doi:10.2307/2589182, if you want to look at it. Another fact about the series, also discussed there, is *b(n)* is the number of ways of writing the integer *n* as a sum of powers of 2, each power being used at most twice (i.e., once more than the legal limit for binary expansions).

Every positive integer *n *appears in the list surrounded by two smaller numbers, which are relatively prime and sum to *n*, exactly once for each such possible distinct sum (counting *m, n, n-m* and *n-m, n, m* as distinct). So one finds 1, 12, 11; 5, 12, 7; 7, 12, 5; and 11, 12, 1; and no other instances of 12 adjacent to smaller numbers. On the other hand, one finds all 46 of 1, 47, 46; 2, 47, 45; 3, 47, 44; … 45, 47, 2; and 46, 47, 1.

Mind you, you have to look through a lot of numbers to find them. After all, 1 appears adjacent to and at position in the series: 1 followed by 2 and 1 is at position 2, 1 then 3 and 2 at position 4, 1 then 4 and 3 at position 8, and so on. 1 followed by 47 and 46 appears at position (!) So if you want to figure out how many numbers are smaller than and relatively prime to 47, examining Stern’s series may not be the best approach.

]]>

From Charlie Drinnan, find the letters’ numbers:

If A, B, C, D and E are all unique digits, what values would work with the following equation?

ABC,CDE × 4 = EDC,CBA

No problem. must be even, and , so . Then so , and so and , or and . But so .

Now so . so .

Finally or , so .

.

This is fairly mechanical although I tripped up several times on the way.

Let be the number of coconuts found by the th pirate, and be the number of coconuts found in the morning. is divisible by 7 so write .

But is what was left when the seventh pirate discarded one coconut out of and hid one seventh of the rest, so . For that to be an integer must be a multiple of 6, so write . Then .

Now . For integer value, , so write . Then .

You see how this goes.

. For integer value, , so write . Then .

. For integer value, , so write . Then .

. For integer value, , so write . Then .

. For integer value, , so write . Then .

. For integer value, , so write . Then . The smallest value corresponds to so .

These pirates were either very sleepy, or incredibly quick at counting coconuts.

]]>Let’s suppose we have three different gap sizes. For instance, the 8 note scale:Here the first note, the note we started with, is F, and the last note added is F♯. That means the Type I rigid gaps are FF♯ and F♯G (the yellow gap on the left, and the purple one immediately to its right, surrounding the last note). AB (the blue gap closest to the center) is a Type II rigid gap, because shifting it by *X* makes it coincide with EF♯, which isn’t a gap, it’s the two gaps surrounding the first note (EF and FF♯). Clearly that means the size of gap AB is the size of EF (which is the same as the size of rigid gap F♯G) plus the size of FF♯. So in this case the size of the largest gap is the sum of the sizes of the two smaller gaps.

But is that always true? Can you get a Type II gap that isn’t the same size as the sum of the gaps surrounding the first point? Or can you get a scale in which the two gaps surrounding the first point are both the size of one or the other of the Type I gaps?

Let’s try to look at all the possibilities. For the Type I (rigid) gaps:

- There are no Type I gaps
- There is only one Type I gap
- There are two Type I gaps, and they’re the same size
- There are two Type I gaps, and they’re different sizes

And for the Type II (rigid) gap:

- There is no Type II gap
- There is a Type II gap, and it is also a Type I gap
- There is a Type II gap, and it is not a Type I gap

Type I gaps surround the last note. If the gaps adjacent to the last note aren’t rigid, that means there’s a note *X* above the last note, which would have to be the first note, and the only way that can happen is if . That is, *X* has to be rational, of the form (in lowest terms). Then the *n* note scale divides the octave into *n* equal intervals, and we have a 1-gap scale. In that case there aren’t any Type II rigid gaps, either. Every gap coincides with a gap when shifted.

If there is no note *X* above the last note, then the gap to the left of the last note is rigid and so is the gap to the right, and there are two Type I gaps *unless those two gaps are one and the same*. In other words, it’s a one-note scale with one (one octave) gap. Which is a degenerate sort of equal division of the octave, so in fact there aren’t any rigid gaps. In other words, if it’s not an equal division of the octave (or a one-note scale), then there must be two distinct Type I gaps. They can be the same size, or not.

Now suppose there is no Type II gap. No gap contains the first note in its shifted interior. The only way that can happen is if there is a note *X* to the left of the first note, and that would have to be the last note. Again, this means it’s an equal division of the octave, and there are no Type I gaps either.

So suppose there is a Type II gap, but it is also a Type I gap. That is, one of its end notes is the last note. Then when shifted, that end will not coincide with a note. If the two Type I gaps are different sizes, then all we can say is we have a 2-gap scale, and there is no particular relationship between the two sizes.

Here gap AB (the blue gap closest to the center) is a Type II rigid gap: if you shift it, it goes from E to where F♯ would be if there were an F♯, and it contains F. It’s also one of the Type I rigid gaps, since B is the last note; the other is BC. So it’s a 2-gap scale.

If the Type II gap is also Type I, and the two Type I gaps are the same size, well… then obviously we have a 1-gap scale, which means an equal division of the octave, but that has no rigid gaps at all, so by contradiction that case is impossible.

Finally, suppose there is a Type II gap, and it is not a Type I gap. That means there’s a note *X* to the right of each of its end notes, so when you shift the gap it’ll coincide with two gaps, the two surrounding the first note. The Type II gap is the sum of the gaps surrounding the first note. One of the two gaps surrounding the first note has its “older” end note (namely the first note) on the right end, the other has the older note on the left end. If you shift the first of these as many times as you can, the “newer” note on the left end reaches the last note first, so this gap matches up with the Type I rigid gap in which the last note is on the left end. But if you shift the second one, the “newer” note on the right end reaches the last note first, so that gap matches up with the Type I rigid gap in which the last note is on the right end. So if the two Type I gaps are different sizes, then so are the two gaps surrounding the first note, and we have a 3-gap scale in which the Type II rigid gap is the same size as the sum of the sizes of the two Type I gaps.

The only way the two gaps surrounding the first note can be the same size is if both Type I gaps are the same size, in which case the Type II gap is exactly twice that size and it’s a 2-gap scale. That would happen, for instance, if instead of ~702 cents, *X* were 700 cents, and you generated an 8-note scale. It would look like the 8-note scale above, except the gaps surrounding the first note both would be 100 cents in size, and so would the gaps surrounding the last note.

Summing up:

- If there are no Type I gaps, there are no Type II gaps, and vice versa; we have a 1-gap scale
- If there is a Type II gap, and it is also a Type I gap, then the two Type I gaps are different sizes, and we have a 2-gap scale, with no particular relationship between the two sizes
- If there is a Type II gap, and it is not a Type I gap, and the Type I gaps are the same size, we have a 2-gap scale with the large gap twice the small gap
- If there is a Type II gap, and it is not a Type I gap, and the Type I gaps are not the same size, we have a 3-gap scale with the large gap the sum of the small gaps

So there you go. I’ve framed this discussion in terms of musical scales, putting notes within an octave, at positions ranging from 0 to 1200 cents, but of course this all could be restated in terms of putting points on a unit line segment, at positions from 0 to 1, or on a circle, at angular positions from 0 to 2π. The Wikipedia article says in the latter form it has applications to phyllotaxis, although I’m not sure it has anything significant to say in that field. The theorem also has applications in the theory of Sturmian words, it says, and if I ever come to grips with what Sturmian words are and why one would care, maybe I’ll write about them here… don’t hold your breath, though…

]]>Here’s the 6-note scale again:

We’re going to look for gaps that are “rigid”, by which we mean this: A rigid gap is one that, if shifted (right) by *X*, does not coincide with another gap.

Think about the gap from F to G (the leftmost blue gap). If you shift it (right) by *X*, you get the gap from C to D. Shift gap CD by *X* and you get the gap GA; shift that by *X* and you get the gap DE. But if you shift DE by *X* you don’t get a gap: you get a segment from A to where B would be if there were a B. The gap DE is rigid. EF (the purple gap) is rigid too, because shifting it by *X* doesn’t give you a gap (it goes from where B would be to C). And AC (orange) is rigid, because shifting it by *X* gives you the segment EG, which is two adjacent gaps (EF and FG), not one.

Generally, there are only two ways a gap can be rigid. One is if when you do the shift one or the other of its end notes doesn’t map to a note. That happens only when one or the other of the end notes is the last note added (A in this case), in which case there isn’t a note *X* to the right of it. We’ll call this a Type I rigid gap.

The other way is if when you do the shift there is a note, or more than one note, in the middle of the shifted segment. But that happens only if the note(s) in the middle have no notes *X* to the left of them. Otherwise there’d be one or more notes in the middle of the unshifted gap, and by the definition of a gap, there isn’t. And the only note with no note *X* to its left is the first note (F in this case), so there can be only one note in the middle of the shifted gap and there can be only one rigid gap of this sort. This is a Type II rigid gap.

And that means there are no more than three rigid gaps: two Type I gaps that start or end on the last point, and one Type II whose shifted version contains the first point. All the other gaps are nonrigid. By shifting any one of them one or more times, you can make it coincide with one of the rigid gaps, and so it must be the same size as one of the rigid gaps. Therefore there can be at most three gap sizes.

That’s the first part of the theorem. Second part, coming up soon.

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At least that’s the pattern up through seven notes. Does it continue?

Adding an eighth note, F♯, splits the whole tone between F and G into two semitones — but, contrary to what you might expect, they’re not equal; one is a diatonic semitone and one is a slightly larger chromatic semitone, 104 cents. It’s yellow.So in the eight note scale there are three kinds of gaps again. (And the big one, the whole tone, is the sum of the two others.)

Add notes nine through twelve and you split the remaining whole tones, ending up with a twelve note scale with two kinds of gaps, diatonic and chromatic semitones.

And you can *still* go on. The thirteenth note splits a chromatic semitone into a diatonic semitone plus a Pythagorean comma (which is very small, about 24 cents, and grey), so we have three kinds of gaps again, with the chromatic semitone being the sum of the diatonic semitone and the Pythagorean comma. (In the diagram I’m running out of room, so I just show the sequence number of each note, not the letter name.)

And if you keep going, when you get to the 17th note it splits the last of the chromatic semitones and you have a scale with two kinds of gaps, diatonic semitones and Pythagorean commas. And so on. You can keep this up for weeks if you want to.

What’s intriguing is that the 5-note scale, called a pentatonic scale, is widely used especially in folk music; the 7-note diatonic scale is the basis of most mainstream Western music; the 12-note chromatic scale (in a slightly different tuning) is what you find on a piano keyboard; and while the 17-note scale has no significant role in western music, the 13th century Islamic music theorist Safi al-Din al-Urmawi developed scales based on division of the octave into 17 notes. Note those are all 2-gap scales. Meanwhile the 3-gap scales with 4, 6, 8, 9, 10… notes don’t turn up much at all. Hm.

Now, all of this can be generalized. You can use tempered fifths (as opposed to pure), you can use other intervals like major or minor sixths, pure or not; heck, you can use any interval you want to generate scales. For that matter you can use a tempered octave as your circle, or a perfect twelfth or something else. And if you do you always find the same pattern. Every scale has one, two, or three kinds of gap, and if there are three kinds, the largest gap is the sum of the other two.

Yes, that’s what you observe, but is it *always* true? It is, and that’s the three-gap theorem.

Wikipedia states it as

if one places n points on a circle, at angles of θ, 2θ, 3θ … from the starting point, then there will be at most three distinct distances between pairs of points in adjacent positions around the circle. When there are three distances, the larger of the three always equals the sum of the other two.

An article by Peter Schiu [paywall] gives it as

Let α > 0 be an irrational number and

n> 1. For 1 ⩽m⩽n, order the fractional parts ofmα to form an increasing sequence (b):_{m}

Then there are at most three distinct values in the set of gaps

gdefined by_{m}

Moreover, if there are three values, then the largest one is the sum of the other two.

Despite the musical roots going back to Pythagoras and Safi al-Din al-Urmawi, this theorem wasn’t proved until the late 1950s.

Shall we look at a proof? Sure. In the next part.

]]>In a more conventional music context, think about notes in the range from one note, say F, to the next F an octave higher. Scales, if you will. Here’s a representation of that. Ignore the black box for now and just focus on the white rectangle. That’s an octave, with our first note, F, at the left end.

(These diagrams may be rather small and hard to see; click on them to expand them.) Notes an octave apart are considered to be equivalent for our purposes, so you can think of the left end as being joined to the right.

One way to talk about the distances between musical pitches — intervals, as they’re called — is to use a unit called cents, where 1200 cents make an octave. We’re going to generate a scale based on an interval called a perfect fifth, which is cents — about 702 cents. So we’ll take our note F, shift it to the right by a distance *X *(see arrow below), and call that new note C.

We’ll call the interval between two consecutive notes a gap. Here there are two gaps, one from F to C which is a perfect fifth, 702 cents, and one from C to F which is a perfect fourth, 498 cents. These are shown in red and green respectively, and they add up to one octave, 1200 cents. This is a rather minimalist scale, two notes, with two kinds of gaps. (The number in the black box tells you how many notes are in the scale.)

Now go up a fifth again, starting from C. That puts you past the right end of the octave, but remember we identify the right end with the left end, so we can think of the arrow as going to the right end and then continuing from the left end to the new note, G.

G lands between F and C. It’s 2*X* above F, minus an octave, which works out to 204 cents, an interval called a whole tone. From G to C is 702 cents minus 204 cents or 498 cents, so G splits the perfect fifth from F to C into a whole tone (in blue) plus another perfect fourth (again in green). In this three note scale there again are two kinds of gaps.

Going up another perfect fifth gives us D, between C and F and splitting that perfect fourth into a whole tone and a major third (294 cents). Now we have a four note scale with *three* kinds of gaps. The major third is orange.Notice the big gap (perfect fourth) is equal to the sum of the other two.

The next perfect fifth gives us A, which splits the remaining perfect fourth into a whole tone and a major third. This five note scale is back to having two kinds of gaps.

Add a sixth note, E, and one of the major thirds gets split into a whole tone and a diatonic semitone (90 cents), shown here in purple. There are three kinds of gaps. The big one (major third this time) is again equal to the sum of the other two.

But adding a seventh note, B, splits the other major third into a whole tone and a diatonic semitone, and once again there are two kinds of gaps.

At this point we have a diatonic scale, or in less pedantic terms, a C major scale if you start on C. (Or A minor if you start on A.)

You start to see a pattern. Each scale has at most three kinds of gaps. If there are three kinds, then the largest is equal to the sum of the two smaller ones. When you add a note to such a scale, it splits one of the large gaps into one of each of the two other kinds; eventually you split all the large gaps and are left with a scale having only two kinds of gaps. But maybe we’re getting ahead of ourselves? That’s the pattern so far, but does it continue?

We’ll see in part 2.

]]>Take a positive integer in base and move its last digits to the front. What is the smallest such that when you do this, the result is exactly times (where is in the range )?

Let , where . So the digits of are the final digits of and the initial digits of , and the digits of are the remaining digits of both. Let be the number of digits in . Then we require

.

Solving, or

Now let and let , . Then

and since and are relatively prime, we must have

.

So we must calculate and then, for each possible value of , obtain and look for the least that satisfies the above relation. (The minimal value of is in fact not but , since there can be no carry to an additional digit when is multiplied by .) Then we can compute , and verify that it does indeed have digits. The smallest such and associated solve the problem.

If is prime, then for all possible and the smallest results when . But if has a common factor with some , then is some factor of . Generally , the number of digits in , is on the order of , so when is prime the solution tends to be many orders of magnitude larger than when it is not.

For instance, consider and . For , . In that case some values of have a common factor with . *E.g.*, gives

where

.

The solution is , leading to

, .

But for , is prime. The number of digits in then turns out to be 27, and

.

And for , is prime too. We end up with the 1499 digit solution

.

If you’d asked me to guess, before I worked any of this out, the order of magnitude of the smallest positive integer which is exactly tripled when you move the last three digits to the front, I’m pretty sure I would have seriously underestimated it.

]]>From Joseph Converse, a puzzle of digital manipulation:

Imagine taking a number and moving its last digit to the front. For example, 1,234 would become 4,123. What is the smallest positive integer such that when you do this, the result is exactly double the original number? (For bonus points, solve this one without a computer.)

Write the number (call it ) as , where ; for instance, for 1,234, and . Let be the number of digits in ; then the number resulting from moving the last digit to the front is . We want .

Solving, we get . Now, 19 is prime and so isn’t a multiple of 19, and therefore must be, that is, .

Going ahead and doing the long division ― that is, dividing 100, 1000, 10000… by 19 until a remainder of 2 is found ― we get ; . That has 16 digits, so won’t work; will give us a 17-digit , though. So the smallest number that works is .

We require to have the same number of digits as , and that means the first digit of can’t be 1. So let’s try making it 2. Then that’s also the last digit of .

Now, the last digit of is 4, which is also the second to last digit of . And then the second to last digit of is 8, which must be the third to last digit of . Then the third to last digit of is 6, with a carry. That means the fourth to last digit of is 6 and the fourth to last digit of is 3, with a carry.

And so on, until we reach a place where the digit of is 1 and is 2 without a carry. Stop there. Follow that procedure and you get as before.

Except we don’t know that this is the smallest answer. We need to do the same with last digit of running from 3 to 9. In fact (and as you’d expect having seen the other method) this is the smallest.

I’m rather surprised the second way didn’t occur to me sooner; it seems a good deal more obvious in retrospect. Then again, had I done it first I think I might’ve looked at the size of the result and been unconfident I hadn’t overlooked something that might give a smaller answer. The first method confirms it and makes it more clear, I think, that this really is the minimum. It also provides the insight that the answer is not just a mysterious sequence of digits but is connected with the decimal representation of 1/19.

Instead of moving the digit to get multiplication by 2, what about by 3? Or by generally? Adapting either method, one finds moving the last digit of 1034482758620689655172413793 to the front gives triple that number. For quadrupling we get a much smaller answer: 102564 = 410256/4. For quintupling we encounter for the first time the necessity of checking all possible last digits, not just the minimum one: Using 5 for the last digit we find 102040816326530612244897959183673469387755, but with 7 we get 142857! This is related to the fact that if you try the first method, you need , and 49 is *not* prime, so when we need rather than .

Here’s a table for = 1 through 9. I am glad the Riddler puzzle didn’t ask for 6 (by hand)… I did these with a spreadsheet.

1 | 1 |

2 | 105263157894736842 |

3 | 1034482758620689655172413793 |

4 | 102564 |

5 | 142857 |

6 | 1016949152542372881355932203389830508474576271186440677966 |

7 | 1014492753623188405797 |

8 | 1012658227848 |

9 | 10112359550561797752808988764044943820224719 |

Another generalization of the original puzzle is to do it in base instead of base 10. There’s no solution in binary, as you can figure out by trying to adapt either method above, or just by noting that doubling a number in binary means appending a 0 to the right end, so you can’t double a number and get a number with the same number of binary digits (bits). In base 3, starting with final digit 2 for , the final digit for is 1 with a carry of 1; 1 for the next digit of gives 0 for the next digit of with a carry; 0 for gives 1 for ; 1 for gives 2 for and we’re done: . In decimal, that’s 32 and 64. (In higher bases we need to check last digits from 2 to .) Alternatively, we want and so require . works and then . So with , and .

In all bases from 3 through 10:

3 | 1012 |

4 | 102 |

5 | 13 |

6 | 1031345242 |

7 | 103524563142 |

8 | 25 |

9 | 10467842 |

10 | 105263157894736842 |

*Edit to add:*

And if this Python script is bug free, then

is the smallest base 15 number which is multiplied by (decimal) 10 when the last digit is moved to the front. At 148 digits, it’s the longest such number for all bases up to 16 and all factors up to (base-1).

]]>

Take a right triangle with sides , , and . Draw perpendiculars to the hypotenuse at each end and a parallel line through the third vertex to make a rectangle .

Angles and are congruent (alternate interior angles). So are and since both are complementary with . So triangles and are similar; the hypotenuses are in the ratio , so the vertical leg and the horizontal leg . Likewise we find (as it should be) and . But then implies or, multiplying though by , .

(Right away you know this proof fails in non Euclidean geometry, because in the first steps it relies on there being one and only one line parallel to through .)

]]>Here’s a right triangle, with legs and and hypotenuse .Here’s the same triangle scaled up by a factor of , so the legs are and and the hypotenuse is .And here it is twice more, scaled up by factors of …… and .Of course these all are similar triangles.

Now, notice the hypotenuse of the first scaled-up triangle is the same length as one leg of the third, so we can draw them juxtaposed like this:Capital letters label the vertices.

Since the triangles are similar, angles and are congruent, and since they’re right triangles, angles and add to a right angle; therefore angle is a right angle.

Similarly the hypotenuse of the second scaled triangle is congruent to the other leg of the third, so they can be juxtaposed as well:and as before, angle is a right angle. Also, angles and sum to a right angle so angle is two right angles, *i.e.*, , , and are colinear. Therefore is a rectangle, and . But and . So . QED.

What I like about this proof is how easy it is to grasp visually. Most proofs of the Pythagorean Theorem deal with areas, specifically persuading you the areas of two squares sum to the same as the area of a third square, but summing areas isn’t easy to visualize. Here there are no areas, just lengths, and we all can see immediately that is just .

(I saw this proof on, um, some blog recently. And failed to make note of exactly where. Sorry.)

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