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Properties of Angles

Before working through the information below you may wish to review this lesson on measuring angles as well as taking a look at the lesson on adding and subtracting angles.

This lesson will provide information and guidance on:

After reviewing the lessons above you will be ready to read through the information below on angles and their relationships with your children. Discuss these as you go and, when you are ready, try the angle relationships worksheet.

Useful Terms

Parallel Lines – lines that are equidistant from each other and never intersect.

Transversal – a line that intersects two or more other lines.

Adjacent Angles – angles that share a common side and that have a common vertex.

Complementary Angles

Complementary Angles are those which add together to make 90°.

The examples above all show two angles that are complementary. Notice that the angles do not have to be adjacent to be complementary. If they are adjacent then they form a right angle.

Supplementary Angles

Supplementary Angles add together to make 180°

The two angles shown above are supplementary to each other. They add together to give 180°. They can be said supplement each other. Note that, as with complementary angles, they do not need to be adjacent to each other.

Opposite Angles

When to lines intersect they create four angles. Each angle is opposite to another and form a pair of what are called opposite angles.

Opposite angles are sometimes called vertical angles or vertically opposite angles.

Corresponding and Alternate Angles

The example below shows two parallel lines and a transversal (a line that cross two or more other lines). This results in eight angles. Each of these angles has a corresponding angle. Looking at the two intersections, the angles that are in the same relative (or corresponding) positions are called corresponding angles.

Since the two lines are parallel, the corresponding angles are equal.

As Shown below, there are also two pairs of alternate interior angles and two pairs of alternate exterior angles. Notice how the interior angles are in between the two parallel lines and the exterior angles are to the outside.

Since the two lines are parallel, the alternate angles shown above are equal.

The Sum of Interior Angles

illustration of how the three interior angles of a triangle can be arranged to show a total angle of 180 degrees

The sum of the interior angles in a triangle is 180°.

illustration of how the four interior angles of a quadrilateral can be arranged to show a total angle of 360 degrees

The sum of the interior angles in a quadrilateral is 360°.

Try the 180° In a Triangle Experiment which is a 2-page (be careful with the scissors) activity to demonstrate that the sum of the interior angles in a triangle is 180°.

Angle Relationship Worksheet

Have your children try the worksheet below that has questions on angle relationships. After completing it your children will be ready to review the lesson on finding missing angles.

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