Geometry Angles Study Guide: GED Math
Practice problems for these concepts can be found at:
Now you are ready to learn about angles. An angle is formed by two rays that share a common endpoint. One ray forms each side of the angle, and the common endpoint is called the vertex. Here is an example of an angle.
There is more than one way to name an angle. The symbol for an angle is . Here are some ways to name an angle.
- You can use three letters to name an angle. The middle letter represents the vertex. For example, the largest angle (bisected by ray AD) could be named BAC or CAB. In both cases, the A is in the middle because it represents the vertex, or the endpoint, of the two rays that make up the angle.
- You can use only one letter to name an angle if the angle you are naming does not share its vertex point with another angle. For example, the vertex illustration shows an angle that is alone. It does not share its vertex point with any other angle. So, you could name it A.
- You can use a number. Look at the second angle diagram again. Notice that a number is written inside the angle. Sometimes angles will be numbered in this way. When this number does not represent the measurement of the angle, you can use the number to name the angle: 1.
Angles are measured in a unit called the degree. The symbol for a degree is °. You can use an instrument called a protractor to measure an angle
Here's how to use a protractor to measure an angle.
Step 1 Place the cross mark or hole in the middle of the straight edge of the protractor on the vertex of the angle you wish to measure.
Step 2 Line up one of the angle's rays with one of the zero marks on the protractor. (Note that there are two zero marks on the protractor. It doesn't matter which one you use.)
Step 3 Keeping the protractor in place, trace the length of the angle's other ray out to the curved part of the protractor.
Step 4 Read the number closest to the ray. (Notice that there are two rows of numbers on the protractor. Make sure you are measuring from zero.) This is the measure of the angle in degrees.
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