Background Trigonometry is about measurement with triangles. We will start by reviewing/discussing some basics of triangles. We will also discuss some basics from algebra such as the distance between two points, and parallel and perpendicular lines. A Triangle An angle is two rays with a common endpoint (known as the vertex of the angle). The rays are known as the sides of the ang le. A triangle is three (3) noncollinear points (known as the vertices ) and the connecting line segments (known as the sides). We name the sides using the vertices, A, B, C: side AB side AC side BC A Basic Facts for triangles are The sum of the measures of the angles is 180 o . Geometry While discussing basic facts, we hav e the following impo rtant facts from geometry. 1. Vertical angles have equal measures.