Geometry puzzle

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Abdul Alhazred
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Re: Geometry puzzle

Post by Abdul Alhazred »

shemp wrote: Thu Sep 17, 2020 1:38 pm Interesting that ceptimus posted this over at ISF this morning and had two correct answers in less than an hour. I'll draw no conclusions.
It's our old buddy correlation with population size at work.
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DJ
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Re: Geometry puzzle

Post by DJ »

Abdul Alhazred wrote: Thu Sep 17, 2020 3:13 pm
shemp wrote: Thu Sep 17, 2020 1:38 pm Interesting that ceptimus posted this over at ISF this morning and had two correct answers in less than an hour. I'll draw no conclusions.
It's our old buddy correlation with population size at work.

More high school geometry students available? :wink:
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robinson
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Re: Geometry puzzle

Post by robinson »

Math is hard
still working on Sophrosyne, but I will no doubt end up with Hubris
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Re: Geometry puzzle

Post by robinson »

Geometry even harder
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Witness
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Re: Geometry puzzle

Post by Witness »

A back of the envelope calculation gives me 9. If that's correct I'll write up my solution. (If not I'll just hang my head in shame.)
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robinson
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Re: Geometry puzzle

Post by robinson »

I’m an idiot
Last edited by robinson on Sat Sep 19, 2020 6:05 am, edited 1 time in total.
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ceptimus
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Re: Geometry puzzle

Post by ceptimus »

Witness wrote: Thu Sep 17, 2020 10:51 pm A back of the envelope calculation gives me 9. If that's correct I'll write up my solution. (If not I'll just hang my head in shame.)
Yes, 9 is the correct answer.
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Abdul Alhazred
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Re: Geometry puzzle

Post by Abdul Alhazred »

ceptimus wrote: Fri Sep 18, 2020 1:30 am
Witness wrote: Thu Sep 17, 2020 10:51 pm A back of the envelope calculation gives me 9. If that's correct I'll write up my solution. (If not I'll just hang my head in shame.)
Yes, 9 is the correct answer.
One of you please explain how to get there. :)
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Witness
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Re: Geometry puzzle

Post by Witness »

Abdul Alhazred wrote: Fri Sep 18, 2020 12:46 pm One of you please explain how to get there. :)
Here you go:

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robinson
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Re: Geometry puzzle

Post by robinson »

Jesus fucking christ almighty
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robinson
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Re: Geometry puzzle

Post by robinson »

It's like posting the Riemann Hypothesis in a puzzle forum. Then laughing like a maniac later that night.
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ceptimus
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Re: Geometry puzzle

Post by ceptimus »

Here's a simple geometric proof by Pan Narrans (a poster at the Freethought Forum). Pan was actually the first poster to solve my posting of the puzzle, but I posted the puzzle there about the same time as here, and before I did at ISF, so the ISF guys were the fastest. I didn't invent the puzzle by the way - I first saw it posed in a YouTube "recommended for you" still - but I didn't watch the video and solved it myself instead.

geometryPuzzle_3.gif

AD = AE = AG = AK (the radius of the small circle)

similarly

BD = BF = BH = BL (the radius of the large circle)

Triangle ADK is an isosceles triangle, with A as its apex. The same goes for triangle BDL, with apex B. The "bases" of the two isosceles triangles (the non equal sides) are KD and DL. Note that the line AC is the same length as half the base of the small triangle plus half the base of the large triangle

AC = (KD + DL)/2, and we know that KD + DL = 6, so AC = 3

Now AG + AC + BH = 8 (the height of the rectangle)

Subtracting AC = 3 gives AG + BH = 5 (the sum of the radii)

Since AD + BD = AG + BH = 5, we have a 3/4/5 right triangle ABC with a hypotenuse (AB) 5 and one side (AC) 3, the other side (BC) must be 4.

X = AE + BF + BC = AG + BH + BC = 5 + 4 = 9

Pan did a clever proof using trigonometry and simultaneous equations, similar to Witness's, first, but then followed it up with this simpler geometric one.

The way I did it was something of a cheat. I reasoned that the puzzle wouldn't have been set unless it had a neat constant answer - not some horrible formula depending on the sizes of the circles drawn. So I figured I could make the smaller circle as small as I wanted without changing the answer. If you draw it with the small circle diameter zero, then you just have a rectangle with short side 8, and the large circle touching the bottom and right hand side of the rectangle and passing through the top left hand corner. The left hand edge of rectangle cuts a chord length 6 across the circle. Then the 3.4.5 triangle is immediately obvious, and you get the solution in a flash.
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Bruce
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Re: Geometry puzzle

Post by Bruce »

Finally got it. Messy and convoluted, but it works. That was fun.
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Such potential!
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Bruce
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Re: Geometry puzzle

Post by Bruce »

It was quite satisfying to see a 3,4,5 right triangle emerge from my mess. :)
Such potential!
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Abdul Alhazred
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Re: Geometry puzzle

Post by Abdul Alhazred »

Bruce wrote: Sat Sep 19, 2020 12:16 am It was quite satisfying to see a 3,4,5 right triangle emerge from my mess. :)
8)
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"Yes! A BIG REWARD!" ====> Click here to turn in a sicko
The arc of the moral universe bends towards chaos.
People who believe God or History are on their side provide the chaos.
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Re: Geometry puzzle

Post by shemp »

Bruce wrote: Sat Sep 19, 2020 12:16 am It was quite satisfying to see a 3,4,5 right triangle emerge from my mess. :)
Yeah, sometimes they emerge from my ass when I'm dropping a log.
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