Answer:
ABC are the marked vertices (points) of a triangle. They join each line ie) A to B= AB A to C= AC B+C=BC. We can when working on combined lines and shapes call the exterior vertices away from the triangle D and E when making reference to finding measurements of adjoining shapes/lines, but not to ABC. We can also use AB to determine the midway point as a independent or alternative use.
We usually see A on the opposite point of a right angle.
Explanation: