I Trigonometry problem of interest

[Charles Link]

TL;DR Summary

The other day I saw a math problem on line, which they said came from a Polish Olympiad, where you compute the length x of the 3rd side which is the same as the radius, so that the sides of length 2,x, and 11 are inscribed on the arc of a semi-circle. The law of cosines applied twice gives the answer of exactly 7, but the arithmetic is so complex that the integer result is rather surprising.

Seemingly by some mathematical coincidence, a hexagon of sides 2,2,7,7, 11, and 11 can be inscribed in a circle of radius 7. The other day I saw a math problem on line, which they said came from a Polish Olympiad, where you compute the length x of the 3rd side which is the same as the radius, so that the sides of length 2,x, and 11 are inscribed on the arc of a semi-circle. The law of cosines applied twice gives the answer for x of exactly 7, but the arithmetic is so complex that the integer result is rather surprising. Might anyone see some method that simplifies things, or is this perhaps simply a case of mathematical coincidence?

 

[jedishrfu]

I think you mean a 6-sided polygon inscribed in a circle of radius 7.

Folks solve it by determining the central angles and determine if they add up to 360 degrees.

One way to play with the problem is to use GeoGebra to visualize it.

 

[Charles Link]

Writing law of cosines twice $11^2=2r^2(1-\cos(\theta))$ and $2^2=2r^2(1-\cos(120-\theta))$ and trigonometric expansion on $\cos(120-\theta)$, one can eliminate the $\cos(\theta)$, and solve for $r$, but $\sin(\theta)$ shows up in the trigonometric expansion, so that the final algebraic result is a quadratic in $r^2$, and the arithmetic is complex. One gets two solutions for $r$: $r=7$, and an extraneous $r$ just slightly less than 6.

Edit: To provide a little more detail $r^2=(250 \pm \sqrt{1936})/6$.

I anticipate there might be a simpler solution to this that gets the result that $r=7$, but it is not readily apparent to me.

 

[kuruman]

TL;DR Summary: The other day I saw a math problem on line, which they said came from a Polish Olympiad, where you compute the length x of the 3rd side which is the same as the radius, so that the sides of length 2,x, and 11 are inscribed on the arc of a semi-circle. The law of cosines applied twice gives the answer of exactly 7, but the arithmetic is so complex that the integer result is rather surprising.

So what exactly are you given and what are you asked to find?

 

[Charles Link]

So what exactly are you given and what are you asked to find?

The problem is to find x. In the lower semi-circle they had one half of a regular hexagon with all 3 sides of length x, so that says immediately that x=r. In the upper semi-circle they had 3 sides of length 2, x , and 11. The problem is to find x. The correct answer is 7, but I'm looking for a simpler way to do it than the cumbersome way I presented with more detail in post 3.

 

[phinds]

I guess it's too much trouble for you to post a graphic of what you are talking about.

 

[Charles Link]

I guess it's too much trouble for you to post a graphic of what you are talking about.

I am all thumbs with computer graphics. Hopefully the explanation is clear enough to follow. :)
The original problem was on-line on Facebook, but normally the share feature isn't allowed on PF.

 

[FactChecker]

I am all thumbs with computer graphics. Hopefully the explanation is clear enough to follow. :)
The original problem was on-line on Facebook, but normally the share feature isn't allowed on PF.

Can you take a screenshot and copy it here?

 

[kuruman]

Is this it?

 

[Charles Link]

In the original problem, x is in between the 2 and the 11, but mathematically it makes no difference.

Thank you @kuruman :)

and what I wonder, is did the authors of the problem find a mathematical coincidence, or might there be a very simple way to show x=r=7? (In the other semi-circle they had one half of a regular hexagon with all 3 sides =x.) Given that x=r, find x.

So far, especially after working through the trigonometry, algebra, and arithmetic, I see nothing that should indicate the answer would be an integer. :) I even tried eliminating $r^2$ and solving for $\cos{\theta}$, but that came with some very complicated arithmetic. I'm attempting to do this without resorting to the use of a calculator to evaluate coefficients, etc. I did get the correct answer, r=7, by solving for $r^2$, but I'm looking for any possible much simpler solution.

 

[jedishrfu]

I guess it's too much trouble for you to post a graphic of what you are talking about.

Let us not forget the marvelous, dapper Snowmen that Charles has posted in the past.

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When I tested the central angles using 7 for the x and the radius,

it worked out, so there may be a subtle linkage.

 

[jedishrfu]

So, given the picture, if x is a chord and x is the radius, we have an equilateral triangle, reducing the problem to a quadrilateral with three inscribed corners on the circle, where two of the sides are both radii as x. The angle between the two x's is 120 degrees.

and so then we can divide the quadrilateral into two triangles, where for triangle 2 we have an its central angle relationship: $sin(2*\theta_2)=x$ and $sin(2*\theta_{11})=(11/2)x=$

and separately we have $2*\theta_2 + 2*\theta_{11} = 120$ which reduces to $\theta_2 + \theta_{11} = 60$

Does that make sense?

 

[Lnewqban]

The problem is to find x. In the lower semi-circle they had one half of a regular hexagon with all 3 sides of length x, so that says immediately that x=r. In the upper semi-circle they had 3 sides of length 2, x , and 11. The problem is to find x. The correct answer is 7, but I'm looking for a simpler way to do it than the cumbersome way I presented with more detail in post 3.

Graphically, x = 7.00

 

[Charles Link]

Does that make sense?

You got the right idea, and I was able to solve it. However, I am rather puzzled why the final answer, when the problem is so arithmetically complex in the way that I solved it, has such a simple integer result. Oftentimes that implies there may be some kind of hidden symmetry involved, but in this case, so far I don't see any.

@Lnewqban very good,(post 13), but the largest circle with R=11 doesn't have its center on the other center. Perhaps I'm overlooking something.