Let be a quadrilateral, and let and be points on sides and , respectively, such that . Ray meets rays and at and , respectively. Prove that the circumcircles of triangles , , , and pass through a common point.
Solutions — 2
Solution 1
Let be the second intersection of the circumcircles of triangles and . Because the quadrilateral is cyclic, , or
Because the quadrilateral is cyclic,
By equations () and (*), it follows that triangle is similar to triangle . Hence and . Note also that . Thus, triangle is similar to triangle . Another way to say this is that there is a spiral similarity centered at that sends triangle to triangle , which implies that there is also a spiral similarity, centered at , that sends triangle to triangle , and vice versa. In terms of complex numbers, this amounts to saying that
Because , points and are obtained by extending corresponding segments of two similar triangles and , namely, and , by the identical proportion. We conclude that triangle is similar to triangle , implying that triangle is similar to triangle . Therefore, as shown before, we can establish the similarity between triangles and , implying that
Solution 2
We will give a solution using complex coordinates. The first step is the following lemma.
Lemma Suppose and are real numbers and , and are complex. The circle in the complex plane passing through , and also passes through the point , independent of .
Proof: Four points and in the complex plane lie on a circle if and only if the cross-ratio
is real. Since we compute
the given points are on a circle.
Lay down complex coordinates with and and on the positive real axis. Then there are real and with , and and hence gives
The line consists of all points of the form for real . Since lies on this line and has zero imaginary part, we see from that it corresponds to . Thus
Apply the lemma with , , , and . Setting gives
and setting gives
Therefore the circumcircles to and meet at
This last expression is invariant under simultaneously interchanging and and interchanging and . Therefore it is also the intersection of the circumcircles of and .