Some definitions are important for angle measurements such as right, obtuse, and acute angles. In this post we are going to talk about these concepts. Adjacent supplementary angle Definition: Given a ray $\overrightarrow{oa}$ and an opposite ray $\overrightarrow{oc}$. We project a ray $\overrightarrow{ob}$ at a point in the plane. We say that the angle formed by points $b\hat{o}c$ is supplementary (supplement) and adjacent (neighbor) to the angle formed by points $a\hat{o}b$.
Angle comparisons are important for properties that will be used in later posts. Congruence Congruence in angles is a concept that is similar to equality of numbers. Here congruence is used with the symbol ($\equiv$) and this primitive notion satisfies the postulates: Reflective every angle is congruent to itself. $\widehat{ab} \equiv \widehat{ab}$ Symmetric If $\widehat{ab} \equiv \widehat{cd}$ then $\widehat{cd} \equiv \widehat{ab}$ Transitive If $\widehat{ab} \equiv \widehat{cd}$ and $\widehat{cd} \equiv \widehat{ef}$ then $\widehat{ab} \equiv \widehat{ef}$