Inscribe a rhombus in a triangle

Geometry Level 4

Using a collapsible compass and a straightedge, how many moves are required to construct an inscribed rhombus in a triangle, given they both share an angle A A ?


Please refer to the terminology in this note for further definitions. See other problems in this set .

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1 solution

Sharky Kesa
Nov 25, 2016

Let the other vertices of the triangle be B B and C C .

  1. Draw a circle centred at A A of arbitrary radius such that it intersects A B AB and A C AC at P P and Q Q . (1 move)

  2. Draw circles centred at P P and Q Q passing through A A . Let these circles intersect each other again at R R . (3 moves)

  3. Draw A R AR let it intersect B C BC at E E . (4 moves)

  4. Draw a circle centred at A A passing through E E . Draw a circle centred at E E passing through A A . Let these two circles intersect each other at M M and N N . (6 moves)

  5. Draw M N MN . Let this line intersect A B AB and A C AC at D D and F F .

  6. Draw D E DE and E F EF .

Here, A D E F ADEF is a rhombus, sharing an angle with triangle A B C ABC , and it took 9 moves.

How do we ensure that this is the minimum number of moves required?

(A solution's worst nightmare)

Manuel Kahayon - 4 years, 6 months ago

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Proof by frequency? I am pretty sure there are no faster solutions, though I could be mistaken. But so far, the best is 9 moves.

Sharky Kesa - 4 years, 6 months ago

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What is proof by frequency?

Never expected to hear a new math term for today...

Manuel Kahayon - 4 years, 6 months ago

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@Manuel Kahayon It's a term I made up. Basically, proof by the number of people that have gotten the same result as me. :P :P

Sharky Kesa - 4 years, 6 months ago

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@Sharky Kesa HAHA nice term :p

My proof is proof by Euclidea TT.TT

Can't progress past delta. OCD does not allow me to skip a level without getting 3 stars on it.

Manuel Kahayon - 4 years, 6 months ago

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@Manuel Kahayon I know what you mean. I completed the game yesterday. I got a quicker solution for Eta level 3 (construct a 7 5 75^{\circ} angle). The problem has 4L as optimal. I got 3L. Feel really proud. They're gonna change it the next update.

Sharky Kesa - 4 years, 6 months ago

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@Sharky Kesa WHOAH that's amazing...

Did the game glitch out or what?

Manuel Kahayon - 4 years, 6 months ago

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@Manuel Kahayon No, it's a legit solution.

Sharky Kesa - 4 years, 6 months ago

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@Sharky Kesa Yeah, draw 2 circles for the 30 degree, then bisect it or something

But, what I meant was, what happened to the game?

Manuel Kahayon - 4 years, 6 months ago

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@Manuel Kahayon Nothing... It works fine for me.

Sharky Kesa - 4 years, 6 months ago

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@Sharky Kesa Oh... That was disappointing... I thought there was going to be something like:

"Congratulations, you are smarter than the game developers and we have decided to give you a cookie"

Or something :p

Manuel Kahayon - 4 years, 6 months ago

Euclidea now lists an eight move solution.

Paolo Esquivel - 3 years, 9 months ago

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Huh, I'll update it as soon as I find it.

Sharky Kesa - 3 years, 9 months ago

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