There are many ways to prove the isosceles triangle theorem. We have SAS for triangle congruence so far, so we will use that.
Note that we have proven SAS by rigid motions so take it as a theorem and not a postulate. In high school geometry, particularly older textbooks and class formats, it is taken as a postulate (assumed to be true) rather than proven as a theorem.
Note that we have proven SAS by rigid motions so take it as a theorem and not a postulate. In high school geometry, particularly older textbooks and class formats, it is taken as a postulate (assumed to be true) rather than proven as a theorem.