How to name a triangle on the basis of its points and Which side of a triangle should be taken as the first, second and third sides for theorem? I wanted to know what sides should be taken as first, second, and third in a triangle for theorems such as the appollinius theorem
And how to name a triangle or any polygon on the basis of its point's location thanks.
 A: Let's say there is a polygon. We usually begin with a vertex named $A$, and then go clockwise or anticlockwise, naming the vertices $B, C$ and etc. If there is a quadrilateral $ACBD$, it is not very standard naming, but if there is a quadrilateral $ADCB$, it is okay, because if you look at it from another direction (e.g. clockwise instead of anticlockwise or vice versa), it would be the letters in order.
Some letters are also not usually used in naming vertices of a polygon. $A, B, C, D, P, Q, R, M$ and $N$ are the most common letters used in naming points, but sometimes there may be $G$ for the centroid, $H$ for the orthocenter, $I$ for the incenter and $O$ for the circumcenter of a polygon (usually a triangle). Letters with the prime symbol, and subscripts may also be used, such as $A^\prime$ and $A_1$, for similar polygons.
Also, as George Ivey said in the comments, usually in a triangle, the line opposite a vertex would be named a lower case letter version of the vertex's name. So, in a triangle $ABC$, usually sides are named like $AB=c$, $BC=a$ and $CA=b$.
