Let be a cyclic quadrilateral with circumcircle and circumradius . The diagonals and intersect at . Suppose that . Let be the foot of the perpendicular from to the line . Point lies on line such that and lie on the line in that order. Let the line perpendicular to from intersect the line perpendicular to from at . Prove that lies on .
Solutions — 2
Solution 1
First observe that
so . Thus there is a symmetry in the problem statement swapping . Let be the centre of and let be the reflection of in which, by
lies on . We claim the two lines concur at . By the symmetry noted above, it suffices to prove that and then will follow by symmetry. We have and
Hence . Thus
giving as required.
Solution 2
As in Solution 1, we prove that and note the symmetry in the problem statement swapping .
Let be the circumcircle of . Since and , the radius of is equal to that of . We have that
This, combined with being equal to the common circumradius of and , means that is the circumcentre of . Let the perpendiculars to , from , intersect at then we have
Combining these
which gives that lies on .
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