A line that has one defined endpoint is called a ray and extends endlessly in one direction. A ray is named after the endpoint and another point on the ray e.g.

Endpoint
AB

The angle that is formed between two rays with the same endpoint is measured in degrees. The point is called the vertex
Angle
The vertex is written as
CAB

In algebra we used the coordinate plane to graph and solve equations.  You can plot lines, line segments, rays and angles in a coordinate plane.
coordinate plane
In the coordinate plane above we have two rays
BAandBD

That form an angle with the vertex in point B.
You can use the coordinate plane to measure the length of a line segment. Point B is at (-2, -2) and C (1. -2). The distance between the two points is 1 - (-2) = 3 units.
Angles can be either straight, right, acute or obtuse.
Straight angles
An angle is a fraction of a circle where the whole circle is 360°. A straight angle is the same as half the circle and is 180° whereas a right angle is a quarter of a circle and is 90°.
You measure the size of an angle with a protractor.
protractor
Two angles with the same measure are called congruent angles. Congruent angles are denoted as
AB

Or could be shown by an arc on the figure to indicate which angles that are congruent.
Congruent angles
Two angles whose measures together are 180° are called supplementary e.g. two right angles are supplementary since 90° + 90° = 180°.
Two angles whose measures together are 90° are called complementary.
Supplement Complement
mA+mB=180


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