Thales is credited with the following five theorems of geometry:
1. A circle is bisected by its diameter.
2. Angles at the base of any isosceles triangle are equal.
3. If two straight lines intersect, the opposite angles formed are equal.
4. If one triangle has two angles and one side equal to another triangle, the two triangles are equal in all respects. (SeeCongruence)
5. Any angle inscribed in a semicircle is a right angle. This is known as Thales' Theorem