Geometry Basics

A point is a location in space that has no size, no width, no length, and no depth. It is represented by a dot and named with a capital letter.

A line is a straight one-dimensional figure that extends infinitely in both directions. It has no thickness and is defined by any two points on it.

A ray is a part of a line that has one fixed endpoint and extends infinitely in one direction. It is named by its endpoint and another point on the ray.

A line segment is a part of a line bounded by two distinct endpoints. Unlike a line, it has a definite length that can be measured.

A plane is a flat, two-dimensional surface that extends infinitely in all directions. It is defined by three non-collinear points.

Points are collinear if they all lie on the same straight line. Any two points are always collinear, but three or more points may or may not be.

Points are coplanar if they all lie on the same plane. Any three points are always coplanar, but four or more points may or may not be.

A convex shape has all interior angles less than 180°, and any line segment between two interior points stays inside the shape. A concave shape has at least one interior angle greater than 180°, creating an indentation.

An acute angle is an angle that measures greater than 0° and less than 90°. Examples include 30°, 45°, and 60°.

An obtuse angle is an angle that measures greater than 90° and less than 180°. An example is 120°.

A right angle measures exactly 90°. It is often indicated by a small square symbol at the vertex.

Complementary angles are two angles whose measures add up to exactly 90°. For example, 35° and 55° are complementary.