Right triangle
Thales' theorem

Thales' theorem states that if B C {\displaystyle BC} is the diameter of a circle and A {\displaystyle A} is any other point on the circle, then △ A B C {\displaystyle \triangle ABC} is a right triangle with a right angle at A. {\displaystyle A.} The converse states that the hypotenuse of a right triangle is the diameter of its circumcircle. As a corollary, the circumcircle has its center at the midpoint of the diameter, so the median through the right-angled vertex is a radius, and the circumradius is half the length of the hypotenuse.
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