Circle inside a hexagon

Geometry Level 3

A circle is inscribed in a hexagon, as shown in the diagram.

Is it possible that the side lengths of the hexagon are 7 , 9 , 11 , 13 , 15 , 17 7,9,11,13,15,17 in some order?

Yes, it's possible No, it's not possible

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1 solution

Áron Bán-Szabó
Jul 16, 2017

Suppose yes. Then we will use the figure below.

x + y + z + v + w + u = ( x + y ) + ( y + z ) + ( z + v ) + ( v + w ) + ( w + u ) + ( u + x ) 2 = 7 + 9 + 11 + 13 + 15 + 17 2 = 36 x+y+z+v+w+u=\dfrac{(x+y)+(y+z)+(z+v)+(v+w)+(w+u)+(u+x)}{2}=\dfrac{7+9+11+13+15+17}{2}=36

From that A B + C D + E F = x + y + z + v + w + z = 36 AB+CD+EF=x+y+z+v+w+z=36 which is impossible, since the sum of three odd integers is always an odd number. (Note: The 7 , 9 , 11 , 13 , 15 , 17 7,9,11,13,15,17 are all odd integers.)

Therefore it is impossible.

Moderator note:

This problem is extremely similar in concept to the first problem from the Basic set for this week . In that problem, we wanted to know if it was possible for x + y y + z z + x \begin{aligned} x &+ y \\ y &+ z \\ z &+ x \end{aligned}

to all be odd. With this problem, we want

x + y y + z z + v v + w w + u u + x \begin{aligned} x &+ y\\ y &+ z\\ z &+ v\\ v &+ w\\ w &+ u\\ u &+ x \end{aligned}

to all be odd. The extra condition that x + y + z + v + w + z x + y + z + v + w + z is an odd number causes this case to fail.

Did you mean AB + CD + EF = x + y +z+ v + w + u (rather than z) = 36 ?

Pedro HK - 3 years, 10 months ago

As always a very good solution. I solved it by taking its converse true and then reaching an obvious result..

Utkarsh Kumar - 3 years, 10 months ago

How do you know that your beginning diagram is true and that two of the line segments don't overlap?

Guy MI - 3 years, 10 months ago

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I'm not sure what you mean here; we're supposing it's true. We also know the congruences x = x, y = y, etc. are true because both line segments are tangent to a circle from a point outside of it.

Jason Dyer Staff - 3 years, 10 months ago

Same method, is this problem an original? It's nice

Dan Ley - 3 years, 10 months ago

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I believed it's a revised version I saw in a recent math Olympiad test.

Pi Han Goh - 3 years, 10 months ago

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Ah thanks, it's nice when problems combine geometry with number theory:)

Dan Ley - 3 years, 10 months ago

where does the equation AB+CD+EF = half the circumference come from?

Kevin Lehmann - 3 years, 10 months ago

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