Given is circle with diameter . Point lies inside the circle, not on line . The line intersects again at . The tangent to at intersects the line through perpendicular to at . Point lies on such that is a tangent, with . Prove that , and lie on a line.
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Solution
Let be the center of and let be the intersection of with . We first prove that . If and coincide, there is nothing to prove. If and do not coincide, then , so or is a cyclic quadrilateral. (In fact, also lies on the corresponding circumscribed circle.) From this, it follows that . We now have in all cases . Since and are tangents to , , so . By the central angle theorem applied to , this angle is also equal to . Altogether, we find
which implies that is a cyclic quadrilateral. Therefore, . Furthermore, by Thales' theorem, , so , which means that , , and lie on a straight line.
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