Let be the centre of the circle . Let be the circle centered at and passing through . The circles and meet at and . The circle intersects the segment at . The line intersects the circle at and . The line intersects the circle at and . Let be the midpoint of the segment . Prove that the lines and divide the angle into three equal parts.
Solution
Write . Since is a cyclic quadrilateral, we have . The central angle in the circle is twice the size of the inscribed angle , so .

The segment is perpendicular to the segment and the quadrilateral is a deltoid. So,
Since is a cyclic quadrilateral, we have . Since is the midpoint of the segment , we have . This implies and
We have shown that , so the lines and divide the angle into three equal parts.
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