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PrintSELECTION EXAMINATION
Greece geometry
Problem
Let an acute angled scalene triangle with . Let be the midpoints of the sides , respectively, and let be altitudes. At the extension of , to the part of , we consider a point , such that the parallel from to intersect the extensions of and at points and , respectively. If the circumcircle of the triangle , say , intersects the line at point and the circumcircle of the triangle , say , intersects the line at point , prove that: . (E. Psychas)
Solution
Since are the midpoints of the sides , respectively, the quadrilaterals and are parallelograms. From and we conclude that: and . From the cyclic quadrilateral () we get: .
From the last three equalities of angles we arrive at the equality , and from this we conclude that the points , are cocyclic.
In a similar fashion, since and we have the equalities and . Also, from the cyclic quadrilateral (since ) we have that: . From the last three equalities of angles we have , and from this we conclude that the points , are cocyclic.
Since the points belong to the circle , from the cyclic quadrilateral we have: . Since the points belong to the circle , from the cyclic quadrilateral we have: . From the last two equalities we get:
The quadrilateral is inscribed to the circle , and hence:
The angle is an external angle of the triangle , and so: The quadrilateral is inscribed into the circle and since it is isosceles trapezium and so .
From the last three equalities of angles we arrive at the equality , and from this we conclude that the points , are cocyclic.
In a similar fashion, since and we have the equalities and . Also, from the cyclic quadrilateral (since ) we have that: . From the last three equalities of angles we have , and from this we conclude that the points , are cocyclic.
Since the points belong to the circle , from the cyclic quadrilateral we have: . Since the points belong to the circle , from the cyclic quadrilateral we have: . From the last two equalities we get:
The quadrilateral is inscribed to the circle , and hence:
The angle is an external angle of the triangle , and so: The quadrilateral is inscribed into the circle and since it is isosceles trapezium and so .
Techniques
Angle chasingCyclic quadrilaterals