Problem:
Let be the orthocentre of an acute triangle with , inscribed in a circle . The circle with centre and radius intersects at the point , which is on the arc not containing . The circle with centre and radius intersects the segment at the point . The line parallel to through , intersects at point . If is the midpoint of and is the foot of the perpendicular from to , prove that the line bisects the segment .
Solution
Solution:
We use standard notation for the angles of triangle . Let be the midpoint of and the centre of . As
the quadrilateral is cyclic. From the relation we get , so
where . From the relation we get
and thus from the triangle we obtain
which means that is the angle bisector of , thus . Moreover, from the fact that and the isosceles triangle has , we get . It follows that,
On the other hand, it is known that and , so is a parallelogram and
Now from (3) and (4) we obtain that
which means that the points are collinear.
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