Two semicircles are tangent to each other’s diameter. Are the line segments connecting their corners parallel?

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## Solution

The green vertices are the tangency points and the circle centres, with right angles at the tangency points and supplementary angles at the circle centres. The red and yellow quads have vertices at the diameter ends and tangency points.

The three dashed quads are all cyclic – green has two opposite right angles, so the angles marked black are equal, so the ones marked red are also equal (isosceles triangles); so the red quad is cyclic & similarly for yellow.