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Quadrilateral with given angles

Mathematics Asked by Sal.Cognato on February 12, 2021

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We are looking for angles x and y.

I have found the values of the following angles:
BEA = 74,
BDA = 64,
ACD = 68,
ECD = 112,

plus the relationship $x+y = 68$.

All other angles equations, from triangles or the sum of angles in the quadrilateral (360) end up in the same equation!

I have found through Geogebra that $x=18$ and $y=50$ but I can’t figure out a second relationship to determine them geometrically!

Does anyone have any ideas?

Thank you!

4 Answers

If you are looking for the proof by elementary geometry, please see Hiroshi Saito(斉藤浩)'s great work about generalized Langley's problem ( https://www.gensu.co.jp/saito/challenge/pdf/3circumcenter_d20180609.pdf ). He introduced the amazing skill named "3 circumcenter method" invented by Ms. aerile_re(pen name), and you can find the solution of this problem in the article (Q1).

Correct answer by Ichungchen on February 12, 2021

Answered by David Dirkse on February 12, 2021

Sinx / Sin(x + 64) = Sin38 / Sin84 *Sin22 / Sin48

Multiply both denom and nom by 4Sin82

Sinx /Sin(x + 64) = 4Sin22Sin38Sin82 / (Cos6Sin48Sin82)

(4Sin22Sin38Sin82 = Sin66 REPLACEMENT)

Sinx / Sin(x + 64) = Cos24 / (4Sin48Cos6Sin82)

Sinx / Sin(x + 64) = 1 / (8Sin24Cos6Sin82)

(2Sin24*Cos6 = Sin54 REPLACRMENT)

Sinx / Sin(x + 64) = 1 / (4Sin54*Sin82)

(1/4Sin54 = Sin18 REPLACEMENT)

Sinx / Sin(x + 64) = Sin18 / Sin82

X = 18

Y = 50

Answered by Hakan Ersöz on February 12, 2021

A proof by Sine Law goes as follows:

Considering triangles $AED, BED, ABD, ABE$:

begin{align} frac {sin y}{AD} &= frac {sin 48^circ}{ED}\ frac {sin x}{EB} &= frac {sin 38^circ}{ED}\ frac {sin 46^circ}{AD} &= frac {sin 64^circ}{BA}\ frac {sin 22^circ}{EB} &= frac {sin 74^circ}{BA}\ end{align}

Equating $ED$ in the first two equations we have:

$$frac {EB sin 38^circ}{sin x} = frac {AD sin 48^circ}{sin y}$$

$EB$ and $AD$ can be expressed in terms of $BA$:

$$frac {AB sin 22^circ sin 38^circ}{sin 74^circ sin x} = frac {AB sin 46^circ sin48^circ}{sin 64^circ sin y}$$

Noting that $x = 68^circ - y$,

$$frac {sin 22^circ sin 38^circ}{sin 74^circ (sin 68^circ cos y - cos 68^circ sin y)} = frac {sin 22^circ sin 38^circ}{sin 74^circ sin (68^circ - y)} = frac {sin 46^circ sin48^circ}{sin 64^circ sin y}$$

Rearranging:

$$(sin 22^circ sin 38^circ sin 64^circ + sin 46^circ sin 48^circ sin 74^circ cos 68^circ) sin y = sin 46^circ sin 48^circ sin 74^circ sin 68^circ cos y$$

Giving the expression for $tan y$:

$$frac {sin 46^circ sin 48^circ sin 74^circ sin 68^circ} {sin 22^circ sin 38^circ sin 64^circ + sin 46^circ sin 48^circ sin 74^circ cos 68^circ}$$

WolframAlpha says it is $50^circ$. One could probably reduce the expression above, but right now I don't see how.

Answered by player3236 on February 12, 2021

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