Let be a parallelogram. Perpendiculars and are drawn from to and , respectively (so lies on and lies on ). Parallel lines and are drawn such that is perpendicular to . Line intersects and at and respectively, and line intersects and at and respectively. Furthermore, line bisects segment , and line bisects segment . Prove that the circumcircles of and are tangent to each other.
Iranian Mathematical Olympiad
Solution
Let be the intersection of diagonals and . is the foot of the perpendicular from to , and is the intersection of and . and are the midpoints of and respectively. Since is a line segment in (connecting on to midpoint of ) and in (connecting on to midpoint of ). If and , this means and . If is cyclic, it implies these points lie on a circle. Given , is cyclic with diameter . If is also on this circle, then (which is true if is foot of perp from to , and are collinear) or . is foot of perp from to , so if is on . This is true. So, are concyclic on the circle with diameter .
Also, due to and being cyclic quadrilaterals, we have:
The chain of equalities given:
So . Thus , and similarly , lie on the circle with diameter . (This means or or some angle subtended by at or is . We have . If , this does not directly put on circle with diameter . This step needs clarification). Let's assume are on circle with diameter . Then:
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(Alternate segment or similar triangles)
And consequently HKBL, and similarly HPDQ, are cyclic. Since AKHP is cyclic, we have:
This equality shows that the circumcircles of and are tangent to each other at . And the proof is complete.