Geometry begins with the simplest building blocks: points, lines, rays, and segments — then angles formed when lines meet.
A point is a location with no size. A line extends forever in both directions. A segment has two endpoints. A ray has one endpoint and extends in one direction.
An angle is formed by two rays sharing a vertex. Measured in degrees: acute < 90°, right = 90°, obtuse 90°–180°, straight = 180°, reflex > 180°.
Complementary angles sum to 90°. Supplementary angles sum to 180°. Vertical angles (opposite when two lines cross) are always equal.
An angle can be named by its vertex (∠B), by three points (∠ABC), or by a number. The middle letter is always the vertex.
Acute angle
Q1: Two lines cross forming vertical angles. One angle is 65°. What is the opposite vertical angle?
Q2: Angles that sum to 90° are called:
Q3: An angle measuring 135° is classified as: