We know that rays of sunlight start from the sun.
Can you tell where they end?
Rays of light have no endpoint. They go and go until they hit another object and make it bright.
It's the same with the rays in math.
A ray is a part of a line that starts at exactly one point and goes forever in one direction.
A ray is drawn with a dot at the endpoint and an arrow pointing to the direction that the ray is going.
We cannot measure the length of a ray.
Why? 🤔
Because it goes on and on forever.
To name a ray, we look at its endpoint first.
Then, we just need the name of any other point on the ray.
The name of our ray is ray AB.
The names of rays include the letters of two points. Above them, we add an arrow pointing to the right.
Tip: The first letter is always the name of the endpoint. The order of the letters cannot be swapped.
When you put two rays together, we create an angle.
An angle is made up of two rays that begin at the same point.
The common point where the two rays begin is called the vertex.
The two rays that make up the angle are called the sides. They're also called the arms.
An angle can be named based on the points that form the angle.
The vertex will always be the middle letter.
This angle is called angle XYZ or angle ZYX. It can also be written as ∠XYZ or ∠ZYX. Notice the angle symbol before the letters.
It's also possible to name this angle by just the letter of its vertex. We write it like this: ∠Y.
Angles are measured based on the size of the opening between the rays.
Angles are measured using degrees.
The angle above is about 30 degrees. It can also be written using the degree symbol: 30°.
Tip: This "degrees" unit for measuring angles has nothing to do with °F, or °C. These are different units, but they do use the same little ° symbol.
We measure angles using a protractor.
Each line measures 1 degree.
There are 5 different types of angles.
A right angle is an angle that's exactly 90°.
It almost looks like a capital letter L.
Four right angles can make a square, or rectangle.
An acute angle is any angle less than 90°.
Acute angles are smaller than right angles.
Obtuse angles are angles greater than 90° but less than 180°.
Obtuse angles are bigger than right angles.
A straight angle is an angle that's exactly 180°.
Straight angles form lines.
Any angle greater than 180° and less than 360° is called a reflex angle.
Great job learning about rays and angles.
Now, practice what you learned to help you remember for longer.