How To Use a Protractor

A protractor is a handy mathematical tool that allows you to measure an angle, or to create an angle of a given measurement. This flat, usually plastic device looks like a semi-circle, and is marked with degree measurements. The straight edge is generally marked with centimeters or inches, and can be used as a ruler. The curved portion will tell you the measurement of the angle. Here's how to use a protractor:

  1. Learn about angles. When drawing or measuring angles with your protractor, it's important to know that there are 360 degrees in a full circle. A straight line is 180 degrees, while a corner (like that found on a square) is 90 degrees, and is known in mathematics as a right angle. Angles that open further than this (measuring more than 90 degrees) are obtuse angles, while those opening less than this (fewer than 90 degrees) are called acute angles.
  2. Measure an angle. First, find the center hole in the straight (not curved) part of the protractor. Position this hole over the vertex (point) of the angle you are measuring. Line up the zero on the protractor with one of the angle's sides. Some protractors will have a straight line parallel to the bottom edge of the protractor that should be lined up exactly with the bottom line of the angle you are measuring. Then, find the point where the second side of the angle meets the protractor's curved edge. Read the number written on the curved edge of the protractor at the point where the angle line intersects it. This is the measurement of the angle, given in degrees.
  3. Draw an angle. First, use the straight side of the protractor to draw a straight line which will become one side of your angle. Then, position the protractor's center hole precisely over one end point of the line you have drawn. Line up the zero on the protractor with the line. Determine the desired measure of your angle, find the corresponding marking on the curved section of the protractor, and make a small mark. Use the protractor's straight side to connect this mark with the end point of the first line (where the center hole was positioned). These two lines intersect to form an angle of the desired measurement.
  4. Determine whether angles are equal. Often, a protractor will be used to determine if two angles are congruent, or equal. First, measure the first angle. Then, measure the second angle. Does the protractor read the same number of degrees both times? If so, the angles are the same.

A protractor is a very handy tool, especially for geometry classes. This simple tool is relatively easy to use, and can be utilized for drawing or measuring individual angles or those found in triangles or other shapes.


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