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## Geometry 229+

### Unit 3: Lesson 1

Transformations & congruence# 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.

### Practice

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, degree. We'll use congruence along with other concepts, like the fact that the interior angle measures of a triangle sum to 180, degree, to find missing measurements.

### Practice

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.

## Want to join the conversation?

- For the first practice question,how do we know that line a and line d have alternate interior angles and corresponding angles,and why does it not work for line b and line c?(14 votes)
- Any pair of lines have these angles, so the question is are they congruent (if you use alternate interior or alternate exterior or corresponding)?

or are they supplementary (if you use same side interior or same side exterior)?

For any one line and the transversal, you can use the fact that adjacent angle are supplementary and vertical angles are congruent.

Thus, for b and c, alternate interior angles would be 88 degrees (180-92) and 91 degrees which are not congruent, thus not parallel. For corresponding angles, you would get the same results of 88 and 91 degrees which are not congruent.(0 votes)

- what exactly does congruence mean?(3 votes)
- congruence is to figures as equal is to expressions. If two things are congruent, they are the same size and/or the same shape.(7 votes)

- I don't get this. could i get help?(3 votes)
- How do you know which one is parallel and which one is not?(0 votes)
- If you fine two congruent angles, you know it is parallel. Also, you can use a compass. If you're still confused, the next few videos shall help.

-Duskpin the Avatar(1 vote)

- ratatat pum pum ratatatatatatata pum pum boom (im shooting you)(1 vote)
- i am a little confused here. what is congruence exactly??(1 vote)
- Congruence compares two line segments, shapes, or 2-d figures. It two things are exactly the same shape and same size, then they are congruent. If two line segments are congruent, that would mean their lengths would be the same. If two figures are congruent (like congruent triangles), then their angles are the same and their side lengths are the same.(1 vote)

- oranges are blue strawberrys are orange. so by this theory 7+7=90(0 votes)
- So... every color that is orange... it is blue.. now that is just
**mold**.*stares at the orange I was just peeling*(4 votes)

- What provides congruence ?(0 votes)
- How do you know those are parallel and not that there could be more then 2 answers there?(0 votes)
- We can figure out which lines are parallel because of their relationship to a line that intersects both of them. For the example in this article, it is A and D because they both have 91 degrees to the left of the line and 89 to the right.(2 votes)