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. AB.

The angle that is formed between two rays with the same endpoint is measured degrees. The point is called the vertex.

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.

In the coordinate plane in the previous page we have two rays BA and BD 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.

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.

Two angles with the same measure are called congruent angles. Congruent angles are denoted as A = B. Or could be shown by an arc on the figure to indicate which angles that are congruent.

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.

m A = M b = 180°

Video lesson: Measure the size of the angle.

To see the solution look at the video in the next page.

What I’m going to do here is just draw angle very briefly and then show you how to use protractor to measure that angle. So say we need to draw two angles, it may depend on the problem that you're working on. Let's just start by drawing some straight line here and then we’ll draw a second line here so it's a bit random. We want to signify this angle by drawing this arc here and then to find the angle we use our protractor and we put a right at the intersection of the two lines and we see we have this crosshair here and we want to line up just perfectly there. So we see that this lines up with the crosshairs we also need this straight line to line up with this line here. Ok, then the next thing we need to do is make sure we don’t measure from the side but from this side so even though it says it's about 150 really its closer to 30. We’re going from 0 up towards 30. Then we look a little more closely see this is 25, 26, 27. So it’s almost exactly 27 degrees.