Amazing Triangle Problem

Here is another simply amazing problem from Five Hundred Mathematical Challenges:

“Problem 154. Show that three solutions, (x1,.y1), (x2,.y2), (x3, y3), of the four solutions of the simultaneous equations
____________(x – h)² + (y – k)² = 4(h² + k²)
______________________xy = hk
are vertices of an equilateral triangle. Give a geometrical interpretation.”

Again, I don’t see how anyone could have discovered this property involving a circle, a hyperbola, and an equilateral triangle. It seems plausible when h.=.k, but it is not at all obvious for h.≠.k. For some reason, I had difficulty getting a start on a solution, until the obvious approach dawned on me. I don’t know why it took me so long.

See the Amazing Triangle Problem.

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The Train Buffs

Here is another train puzzle, this time from J. A. H. Hunter’s Entertaining Mathematical Teasers:

“Mike had made the [train] trip many times. ‘That’s the morning express from Tulla we’re passing,’ he said. ‘It left Tulla one hour after we pulled out from Brent, but we’re just 25% faster.’ ‘That’s right, and we’re also passing Cove, two-thirds the distance between Brent and Tulla,’ Martin agreed. ‘So we’re both right on schedule.’ Obviously a couple of train buffs! Assuming constant speeds and no stops, how long would it be before they reached Tulla?”

Answer.

See the Train Buffs for a solution.

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One Year Anniversary

It is hard to believe a whole year has passed since I started this blog. What is even more surprising is that by February I thought I was about done. I had more or less uploaded the math curiosities and problems I had been thinking about over the years and had presented most of the math essays I had written. There are of course only a finite number of math problems in the world, so I thought I was about done. But much to my surprise I kept finding one more thing that interested me, either an essay or math problem. So here I am. We will have to see what the next year brings.

What to say on this anniversary? I think I will give a retrospective about how the website has been received this past year. This is a challenge, since virtually all my visitors have been silent (which means I haven’t faced criticism, but then I generally haven’t received the necessary correctives either). There are ways to glean some information about visitors and I extracted what I could from the simple plugin I use to count visitors to different posts. I hear that Google analytics provides lots of details, such as the country of origin of a visitor (which would be interesting) and the like, but I have avoided Google and the other social networks as much as possible. After all, I am only an old curmudgeon with old-school notions of privacy.

See One Year Anniversary

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Geometric Puzzle Madness

I have been subverted again by a recent post by Ben Orlin, “Geometry Puzzles for a Winter’s Day,” which is another collection of Catriona Shearer’s geometric puzzles, this time her favorites for the month of November 2019 (which Orlin seems to have named himself). I often visit Orlin’s blog, “Math with Bad Drawings”, so it is hard to kick my addiction to Shearer’s puzzles if he keeps presenting collections. Her production volume is amazing, especially as she is able to maintain the quality that makes her problems so special.

The Stained Glass puzzle generated some discussion about needed constraints to ensure a solution. Essentially, it was agreed to make explicit that the drawing had vertical and horizontal symmetry in the shapes, that is, flipping it horizontally or vertically kept the same shapes, though some of the colors might be swapped.

See Geometric Puzzle Madness

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Magic Hexagons

This is truly an amazing result from Five Hundred Mathematical Challenges.

“Problem 119. Two unequal regular hexagons ABCDEF and CGHJKL touch each other at C and are so situated that F, C, and J are collinear.

Show that

(i) the circumcircle of BCG bisects FJ (at O say);
(ii) ΔBOG is equilateral.”

I wonder how anyone ever discovered this.

See the Magic Hexagons

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Factory Location Problem

This is a somewhat elegant problem from the 1987 Discover magazine’s Brain Bogglers by Michael Stueben:

“Each dot in the figure at left represents a factory. On which of the city’s 63 intersections should a warehouse be built to make the total distance between it and all the factors as short as possible? (A much simpler solution than counting and totaling the distances is available.)”

Note that the distance is the taxicab distance I discussed in my article South Dakota Travel Problem rather than the distance along straight lines between the warehouse and factories.

Answer.

See the Factory Location Problem for solutions.

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Number of the Beast

If you will pardon the pun, this is a diabolical problem from the collection Five Hundred Mathematical Challenges.

“Problem 5. Calculate the sum

__________

It has a non-calculus solution, but that involves a bunch of manipulations that were not that evident to me, or at least I doubt if I could have come up with them. I was able to reframe the problem using one of my favorite approaches, power series (or polynomials). The calculations are a bit hairy in any case, but I was impressed that my method worked at all.

Answer.

See the Number of the Beast for solutions.

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Movie Projector Problem

Here is another Brain Bogglers problem from 1987.

“Exactly four minutes after starting to run—when the take-up reel was rotating one and a half times as fast as the projecting reel—the film broke. (The hub diameter of the smaller take-up reel is 8 cm and the hub diameter of the projecting reel is 12 cm.) How many minutes of film remain to be shown?”

This feels like another problem where there is insufficient information to solve it, and that makes it fun and challenging. In fact, I was stumped for a while until I noticed something that was the key to completing the solution.

Answer.

See the Movie Projector Problem for a solution.

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