Difference between revisions of "1990 USAMO Problems/Problem 5"
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Let <math>\omega_1</math> be the circumcircle with diameter <math>AB</math> and <math>\omega_2</math> be the circumcircle with diameter <math>AC</math>. We claim that the second intersection of <math>\omega_1</math> and <math>\omega_2</math> other than <math>A</math> is <math>A'</math>, where <math>A'</math> is the feet of the perpendicular from <math>A</math> to segment <math>BC</math>. Note that <cmath>\angle AA'B=90^{\circ}=\angle AB'B</cmath> so <math>A'</math> lies on <math>\omega_1.</math> Similarly, <math>A'</math> lies on <math>\omega_2</math>. Hence, <math>AA'</math> is the radical axis of <math>\omega_1</math> and <math>\omega_2</math>. By the Radical Lemma, it suffices to prove that the intersection of lines <math>MN</math> and <math>PQ</math> lie on <math>AA'</math>. But, <math>MN</math> is the same line as <math>CC'</math> and <math>PQ</math> is the same line as <math>BB'</math>. Since <math>AA', BB'</math>, and <math>CC'</math> intersect at the orthocenter <math>H</math>, <math>H</math> lies on the radical axis <math>AA'</math> and we are done. <math>\blacksquare</math> | Let <math>\omega_1</math> be the circumcircle with diameter <math>AB</math> and <math>\omega_2</math> be the circumcircle with diameter <math>AC</math>. We claim that the second intersection of <math>\omega_1</math> and <math>\omega_2</math> other than <math>A</math> is <math>A'</math>, where <math>A'</math> is the feet of the perpendicular from <math>A</math> to segment <math>BC</math>. Note that <cmath>\angle AA'B=90^{\circ}=\angle AB'B</cmath> so <math>A'</math> lies on <math>\omega_1.</math> Similarly, <math>A'</math> lies on <math>\omega_2</math>. Hence, <math>AA'</math> is the radical axis of <math>\omega_1</math> and <math>\omega_2</math>. By the Radical Lemma, it suffices to prove that the intersection of lines <math>MN</math> and <math>PQ</math> lie on <math>AA'</math>. But, <math>MN</math> is the same line as <math>CC'</math> and <math>PQ</math> is the same line as <math>BB'</math>. Since <math>AA', BB'</math>, and <math>CC'</math> intersect at the orthocenter <math>H</math>, <math>H</math> lies on the radical axis <math>AA'</math> and we are done. <math>\blacksquare</math> | ||
− | {{ | + | ==Solution 4== |
+ | |||
+ | We know that <math>BCB'C'</math> is a cyclic quadrilateral. Hence, | ||
+ | |||
+ | <cmath>HB \cdot HB' = HC \cdot HC' </cmath> | ||
+ | |||
+ | <cmath>\implies Pow_{\omega_{1}} = Pow_{\omega_{2}} </cmath> | ||
+ | |||
+ | <cmath>\implies HM \cdot HN = HP \cdot HQ </cmath> | ||
+ | |||
+ | <cmath>\implies MPNQ is cyclic \blacksquare </cmath>. | ||
== See Also == | == See Also == |
Revision as of 00:40, 27 September 2024
Problem
An acute-angled triangle is given in the plane. The circle with diameter intersects altitude and its extension at points and , and the circle with diameter intersects altitude and its extensions at and . Prove that the points lie on a common circle.
Solution 1
Let be the intersection of the two circles (other than ). is perpendicular to both , implying , , are collinear. Since is the foot of the altitude from : , , are concurrent, where is the orthocentre.
Now, is also the intersection of , which means that , , are concurrent. Since , , , and , , , are cyclic, , , , are cyclic by the radical axis theorem.
Solution 2
Define as the foot of the altitude from to . Then, is the orthocenter. We will denote this point as . Since and are both , lies on the circles with diameters and .
Now we use the Power of a Point theorem with respect to point . From the circle with diameter we get . From the circle with diameter we get . Thus, we conclude that , which implies that , , , and all lie on a circle.
Solution 3 (Radical Lemma)
Let be the circumcircle with diameter and be the circumcircle with diameter . We claim that the second intersection of and other than is , where is the feet of the perpendicular from to segment . Note that so lies on Similarly, lies on . Hence, is the radical axis of and . By the Radical Lemma, it suffices to prove that the intersection of lines and lie on . But, is the same line as and is the same line as . Since , and intersect at the orthocenter , lies on the radical axis and we are done.
Solution 4
We know that is a cyclic quadrilateral. Hence,
.
See Also
1990 USAMO (Problems • Resources) | ||
Preceded by Problem 4 |
Followed by Last Question | |
1 • 2 • 3 • 4 • 5 | ||
All USAMO Problems and Solutions |
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