What is the difference between postulate and theorem

A postulate (or sometimes called as an axiom) is something assumed rather than proven. A theorem is something that is proven. To make a theorem, we first start with undefined terms. Such as line, point, and plane. Then next comes the postulates. Then the definitions. And in some theorems, a previously proven theorem is sometimes used in order to prove the new theorem.

40 & 50 in the right corner are the complementary angles as when placed in a sum (added together) they make 90°