indian and babylonian mathematicians knew this for special cases before thales proved it. it is believed that thales learned that an angle inscribed in a semicircle is a right angle during his travels to babylon. the theorem is named after thales because he was said by ancient sources to have been the first to prove the theorem, using his own results that the base angles of an isosceles triangle are equal, and that the sum of angles in a triangle is equal to 180°.