Geometry · Tangent circles · Angle in a semicircle · Isosceles triangle · Angle bisector
Two circles touch each other internally at a point \(A\). Let \(AB\) be a diameter of the larger circle. Through the other endpoint \(B\) of this diameter a line is drawn which touches the smaller circle at a point \(C\) and meets the larger circle again at a point \(D\).
Prove that the line \(AC\) bisects the angle \(BAD\).
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Serbian Regional Competition 2007, high school grade I, category A, problem 2. Organized by the Mathematical Society of Serbia (DMS). Source