Art of Problem Solving
AoPS Online
Math texts, online classes, and more
for students in grades 5-12.
Visit AoPS Online ‚
Books for Grades 5-12 Online Courses
Beast Academy
Engaging math books and online learning
for students ages 6-13.
Visit Beast Academy ‚
Books for Ages 6-13 Beast Academy Online
AoPS Academy
Small live classes for advanced math
and language arts learners in grades 2-12.
Visit AoPS Academy ‚
Find a Physical Campus Visit the Virtual Campus

Difference between revisions of "2006 Canadian MO Problems/Problem 5"

(→‎Problem)
(→‎Solution)
Line 9: Line 9:
  
 
==Solution==
 
==Solution==
−
{{solution}}
+
Let the intersection of the tangents at <math>D</math> and <math>E</math>, <math>E</math> and <math>F</math>, <math>F</math> and <math>D</math> be labeled <math>Z, X,Y</math>, respectively.
 +
It is a well-known fact that in a right triangle <math>PQR</math> with <math>M</math> the midpoint of hypotenuse <math>PR</math>, triangles <math>MQR</math> and <math>PQM</math> are isosceles.
 +
Now, we do some angle-chasing:
 +
<cmath>
 +
\begin{align*}
 +
\angle{EDF} &= \angle{EDA} + \angle{ADF} \\
 +
                    &= \angle{XEA} + \angle{AFX} \\
 +
                    &= (180^\circ - \angle{AEX}) + (180^\circ - \angle{YFA}) \\
 +
                    &= 2\angle{FAB} + 2\angle{CAE}\\
 +
                    &= 2(\angle{FAE} - 90^\circ)\\
 +
                    &= 2(90^\circ - \angle{EDF}),
 +
\end{align*}
 +
</cmath>
 +
whence we conclude that <math>\angle{EDF} = 60^\circ.</math>
 +
Next, we will prove that triangle<math>DYF</math> is equilateral. To see this, note that
 +
<cmath>
 +
\begin{align*}
 +
\angle{DYF} &= \angle{FAB} + \angle{BAD} \\
 +
                    &= \angle{FDY} \\
 +
                    &= \angle{YFD}.
 +
\end{align*}
 +
</cmath>
 +
Then, <math>\angle{FED} = 60^\circ</math> as well, and we are done.
  
 
==See also==
 
==See also==

Revision as of 16:59, 3 June 2011

Problem

The vertices of a right triangle $ABC$ inscribed in a circle divide the circumference into three arcs. The right angle is at $A$, so that the opposite arc $BC$ is a semicircle while arc $AB$ and arc $AC$ are supplementary. To each of the three arcs, we draw a tangent such that its point of tangency is the midpoint of that portion of the tangent intercepted by the extended lines $AB$ and $AC$. More precisely, the point $D$ on arc $BC$ is the midpoint of the segment joining the points $D^\prime$ and $D^\prime^\prime$ (Error compiling LaTeX. Unknown error_msg) where the tangent at $D$ intersects the extended lines $AB$ and $AC$. Similarly for $E$ on arc $AC$ and $F$ on arc $AB$. Prove that triangle $DEF$ is equilateral.

Solution

Let the intersection of the tangents at $D$ and $E$, $E$ and $F$, $F$ and $D$ be labeled $Z, X,Y$, respectively. It is a well-known fact that in a right triangle $PQR$ with $M$ the midpoint of hypotenuse $PR$, triangles $MQR$ and $PQM$ are isosceles. Now, we do some angle-chasing: \begin{align*} \angle{EDF} &= \angle{EDA} + \angle{ADF} \\ &= \angle{XEA} + \angle{AFX} \\ &= (180^\circ - \angle{AEX}) + (180^\circ - \angle{YFA}) \\ &= 2\angle{FAB} + 2\angle{CAE}\\ &= 2(\angle{FAE} - 90^\circ)\\ &= 2(90^\circ - \angle{EDF}), \end{align*} whence we conclude that $\angle{EDF} = 60^\circ.$ Next, we will prove that triangle$DYF$ is equilateral. To see this, note that \begin{align*} \angle{DYF} &= \angle{FAB} + \angle{BAD} \\ &= \angle{FDY} \\ &= \angle{YFD}. \end{align*} Then, $\angle{FED} = 60^\circ$ as well, and we are done.

See also

2006 Canadian MO (Problems)
Preceded by
Problem 4 		1 • 2 • 3 • 4 • 5 Followed by
Last question
Retrieved from "https://artofproblemsolving.com/wiki/index.php?title=2006_Canadian_MO_Problems/Problem_5&oldid=39069"