Japanese theorem for cyclic quadrilaterals

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What Wikipedia says

In geometry, the Japanese theorem states that the centers of the incircles of certain triangles inside a cyclic quadrilateral are vertices of a rectangle. It was originally stated on a sangaku tablet on a temple in Yamagata prefecture, Japan, in 1880.

Excerpt from the Wikipedia article “Japanese theorem for cyclic quadrilaterals”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Kmhkmh, CC BY 3.0, via Wikimedia Commons.