What is a triangle in a circle that is formed by two chords and a diameter?

If a triangle is drawn in a circle with a diameter as the base of the triangle, then the angle opposite that diameter is a right angle.

This is an extension of the theorem that the angle which an arc of a circle subtends at the centre of a circle is twice the angle which the arc subtends at the circumference.

In the case of a diameter, then the angle subtended at the centre is 180° and thus the angle at the circumference is 90°.