The unknown term that is being asked for this item is an ANGLE. This defined geometric term is formed when two rays combined at a common point usually called the vertex. The combining rays are called sides. The types of angles are acute, right, obtuse, straight, reflex, etc depending on its measure.
let's notice something, angles α and β are both in the I Quadrant, and on the first quadrant the x-coordinate/cosine and y-coordinate/sine are both positive.