一、(50 分)As shown in Figure 1, quadrilateral is inscribed in a circle. The extensions of and intersect at , and the extensions of and intersect at . is any point on the circle, and and intersect the circle at and , respectively. If the diagonals and intersect at , prove that , , and are collinear.
Solution
Connect , ,
, , ,
. From
,
,
we have ,
Multiplying the two equations, we get .
Furthermore, from , , we have , .
Multiplying these two equations, we get .
From (1) and (2), we have .
Thus, .
By Menelaus' theorem, we have
From (3) and (4), we get .
Therefore, , , and intersect at one point.
That is, , , and are collinear.
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