Difference between revisions of "1993 USAMO Problems/Problem 2"
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Let <math>X</math>, <math>Y</math>, <math>Z</math>, <math>W</math> be the foot of the altitude from point <math>E</math> of <math>\triangle AEB</math>, <math>\triangle BEC</math>, <math>\triangle CED</math>, <math>\triangle DEA</math>. | Let <math>X</math>, <math>Y</math>, <math>Z</math>, <math>W</math> be the foot of the altitude from point <math>E</math> of <math>\triangle AEB</math>, <math>\triangle BEC</math>, <math>\triangle CED</math>, <math>\triangle DEA</math>. | ||
− | Note that reflection of <math>E</math> over the | + | Note that reflection of <math>E</math> over all the points of <math>XYZW</math> is similar to <math>XYZW</math> with a scale of <math>2</math> with center <math>E</math>. Thus, if <math>XYZW</math> is cyclic, then the reflections are cyclic. |
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Revision as of 07:09, 10 April 2023
Contents
[hide]Problem 2
Let be a convex quadrilateral such that diagonals and intersect at right angles, and let be their intersection. Prove that the reflections of across , , , are concyclic.
Solution
Diagram
Work
Let , , , be the foot of the altitude from point of , , , .
Note that reflection of over all the points of is similar to with a scale of with center . Thus, if is cyclic, then the reflections are cyclic.
is right angle and so is . Thus, is cyclic with being the diameter of the circumcircle.
Follow that, because they inscribe the same angle.
Similarly , , .
Futhermore, .
Thus, and are supplementary and follows that, is cyclic.
See Also
1993 USAMO (Problems • Resources) | ||
Preceded by Problem 1 |
Followed by Problem 3 | |
1 • 2 • 3 • 4 • 5 | ||
All USAMO Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.