If a right triangle is inscribed in a circle, then the hypotenuse is a diameter. Conversely, if one side of an inscribed triangle is a diameter of the circle, then the triangle is a right triangle and the angle opposite the diameter is the right angle.
The Inscribed Right Triangle Theorem is also know as the Thales' Theorem.
Consider a right triangle with inscribed in a circle.
Substituting into the equation above gives that Then, is a semicircle, implying that the hypotenuse of is a diameter of the circle.
For the converse, consider a triangle inscribed in a circle such that one side of the triangle is a diameter of the circle.
Since is a diameter, then is a semicircle and then Now, the Inscribed Angle Theorem gives a relation between this arc and the angle opposite to the diameter. The last equation implies that is a right angle which makes a right triangle.