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Getting ready for congruence

Finding missing triangle angle measures and identifying parallel lines from angle measures on transversals help prepare us to learn about congruence.
Let’s refresh some concepts that will come in handy as you start the congruence unit of the high school geometry course. You’ll see a summary of each concept, along with a sample item, links for more practice, and some info about why you will need the concept for the unit ahead.
This article only includes concepts from earlier courses. There are also concepts within this high school geometry course that are important to understanding congruence. If you have not yet mastered the Intro to Euclidean geometry lesson, the Rigid transformations overview lesson, or the Properties & definitions of transformations lesson, it may be helpful for you to review those before going farther into the unit ahead.

Using angle relationships

What is this, and why do we need it?

When lines intersect, especially when a transversal intersects a pair of parallel lines, the intersections form angles with special relationships. We will use these angle relationships to explain how to construct parallel or perpendicular lines, which in turn help us to bisect angles and line segments. We will also use parallel sides to reveal more properties of parallelograms.


Problem 1
Exactly two of the lines in the following figure are parallel.
The figure is not to scale.
Which two lines are parallel?
Choose 2 answers:

For more practice, go to Angle relationships with parallel lines.

Where will we use this?

Here are a few of the exercises where reviewing angle relationships might be helpful:

Finding missing angle measures in triangles

What is this, and why do we need it?

The three angle measures in any triangle add up to 180°. We'll use congruence along with other concepts, like the fact that the interior angle measures of a triangle sum to 180°, to find missing measurements.


Problem 2
Find the value of x in the triangle shown below.
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi

For more practice, go to Find angles in triangles.

Where will we use this?

Here are the first few exercises where reviewing angle measures in triangles might be helpful.

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