一、(40 points) As shown in Figure 1, in the cyclic pentagon , , the diagonals and intersect at point , and a point is taken on the extension of . Prove that the necessary and sufficient condition for points and to be concyclic is that points and are collinear.
Solution
Connect . From , we know
and the extension of intersects the extension of .
When , , and are collinear, the extension of intersects the extension of at point . Then
.
Thus,
.
Also,
.
Therefore, .
Hence, , , , and are concyclic.
When , , , and are concyclic, let the circumcircle of be , then intersects at points and .
If , , and are not collinear, then the intersection point of and the extension of is different from .
From the above proof, , , , and are concyclic, i.e., is also on . Thus, line intersects at three points , , and , which is impossible.
Therefore, , , and are collinear.
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