Stage: 4 Challenge Level: 
A circle touches the lines $OA$ extended, $OB$ extended and $AB$ where $OA$ and $OB$ are perpendicular..
Show that the diameter of the circle is equal to the perimeter of the triangle.
Tangents. Pythagoras' theorem. Long problems. Investigations. Stage 5 needs dealing with. Mathematical reasoning & proof. Circles. Perimeters. Cosine rule. Radius (radii) & diameters.
Published January 1999.
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