8.5: Similar and Congruent Figures
Introduction
The Mathematical Floor
Mrs. Gilman brought a small group of students over to look at this tile floor in the hallway of the art museum.
“You see, there is even math in the floor,” she said, smiling. Mrs. Gilman is one of those teachers who loves to point out every place where math can be found.
“Okay, I get it,” Jesse started. “I see the squares.”
“There is a lot more math than just squares,” Mrs. Gilman said, walking away with a huge smile on her face.
“She frustrates me sometimes,” Kara whispered, staring at the floor. “Where is the math besides the squares?”
“I think she is talking about the size of the squares,” Hannah chimed in. “See? There are two different sizes.”
“Actually there are three different sizes, and there could be more that I haven’t found yet,” Jesse said.
“Remember when we learned about comparing shapes that are alike and aren’t alike? It has to do with proportions or something like that,” Hannah chimed in again.
All three students stopped talking and began looking at the floor again.
“Oh yeah, congruent and similar figures, but which are which?”Kara asked.
What is the difference between congruent and similar figures? This lesson will teach you all about congruent and similar figures. When you are all finished with this lesson, you will have a chance to study the floor again and see if you can find the congruent and the similar figures.
What You Will Learn
By the end of this lesson you will be able to demonstrate the following skills:
- Recognize congruence.
- Find unknown measures of congruent figures.
- Recognize similarity.
- Check for similarity between given figures
Teaching Time
I. Recognize Congruence
In the last lesson we began using the word “congruent.” We talked about congruent lines and congruent angles. The word congruent means exactly the same. Sometimes, you will see this symbol
In this lesson, we are going to use the word congruent to compare figures.
Congruent figures have exactly the same size and shape. They have congruent sides and congruent angles. Here are some pairs of congruent figures.
Compare the figures in each pair. They are exactly the same! If you’re not sure, imagine that you could cut out one figure and place it on top of the other. If they match exactly, they are congruent.
How can we recognize congruence?
We test for congruency by comparing each side and angle of two figures to see if all aspects of both are the same. If the sides are the same length and the angles are equal, the figures are congruent. Each side and angle of one figure corresponds to a side or angle in the other. We call these corresponding parts. For instance, the top point of one triangle corresponds to the top point of the other triangle in a congruent pair.
It is not always easy to see the corresponding parts of two figures. One figure may be rotated differently so that the corresponding parts appear to be in different places. If you’re not sure, trace one figure and place it on top of the other to see if you can make them match. Let’s see if we can recognize some congruent figures.
Example
Which pair of figures below is congruent?
Let’s analyze one pair at a time to see if we can find any corresponding angles and sides that are congruent.
The figures in the first pair appear to be the same shape, if we rotate one
We only know the measure of one angle in the first two figures. We can compare these angles if they are corresponding parts. They are, because if we rotate one figure these angles are in the same place at the top of each figure. Now compare their measures. The angle in the first figure is
The two triangles in the second pair seem to have corresponding parts: a long base and a wide angle at the top. We need to know whether any of these corresponding parts are congruent, however. We know the measure of the top angle in each figure: it is
That leaves the last pair. Can you find the corresponding parts? If we rotate the second figure
8J. Lesson Exercises
Answer true or false for each question
- Congruent figures have the same number of sides and angles.
- Congruent figures can have one pair of angles with the same measure, but not all angles have the same measure.
- Congruent figures can be different sizes as long as the angle measures are the same.
Discuss your answers with a friend. Be sure you understand why each answer is true or false.
II. Find Unknown Measures of Congruent Figures
We know that congruent figures have exactly the same angles and sides. That means we can use the information we have about one figure in a pair of congruent figures to find the measure of a corresponding angle or side in the other figure. Let’s see how this works. Take a look at the congruent figures below.
We have been told these two parallelograms are congruent.
Can you find the corresponding parts?
If not, trace one parallelogram and place it on top of the other. Rotate it until the parts correspond.
Which sides and angles correspond?
We can see that side
What other sides are congruent? Let’s write them out.
We can also write down the corresponding angles, which we know must be congruent because the figures are congruent.
Now that we understand all of the corresponding relationships in the two figures, we can use what we know about one figure to find the measure of a side or angle in the second figure.
Can we find the length of side
We do not know the length of
Now let’s look at the angles. Can we find the measure of
It corresponds to
We were able to combine the given information from both figures because we knew that they were congruent.
Yes and the more you work on puzzles like this one the easier they will become.
8K. Lesson Exercises
Answer this question.
1. What is the measure of
Take a few minutes to check your answer with a friend. Correct any errors and then continue with the next section.
III. Recognize Similarity
Some figures look identical except they are different sizes. The angles even look the same. When we have figures that are proportional to each other, we call these figures similar figures. Similar figures have the same angle measures but different side lengths.
What is an example of similar figures?
Squares are similar shapes because they always have four
Let’s look at some pairs of similar shapes.
Notice that in each pair the figures look the same, but one is smaller than the other. Since they are not the same size, they are not congruent. However, they have the same angles, so they are similar.
IV. Check for Similarity between Given Figures
Unlike congruent figures, similar figures are not exactly the same. They do have corresponding features, but only their corresponding angles are congruent; the corresponding sides are not. Thus when we are dealing with pairs of similar figures, we should look at the angles rather than the sides. In similar figures, the angles are congruent, even if the sides are not.
Notice that one angle in each pair of figures corresponds to an angle in the other figure. They have the same shape but not the same size. Therefore they are similar.
Let’s find the corresponding angles in similar figures.
Example
List the corresponding angles in the figures below.
Angles
How do the angles line up?
Angles
As we’ve said, the sides in similar figures are not congruent. They are proportional, however. Proportions have the same ratio. Look at
The sides from one figure are on the top, and the proportional sides of the other figure are on the bottom.
Example
List all of the pairs of corresponding sides in the figures below as proportions.
Try lining up the figures by their angles. It may help to trace one figure and rotate it until it matches the other.
Which sides are proportional?
\begin{align*}\frac{OP}{RS}\end{align*}
Now that we’ve got one pair, let’s do the same for the rest.
\begin{align*}\frac{NO}{QR}, \frac{MP}{TS}, \frac{MN}{TQ}\end{align*}
Now let’s use what we have learned to check for similarity between figures.
Example
Which pair of figures below is similar?
For figures to be similar, we know that the angles must be congruent and the sides must exist in proportional relationships to each other. Let’s check each pair one at a time.
We only know some of the angles in each triangle in the first pair. They both have a \begin{align*}50^{\circ}\end{align*}
\begin{align*}&\text{Triangle 1} && \text{Triangle 2}\\
&50 + 60 + \text{angle} \ 3 = 180 && 50 + 80 + \text{angle} \ 3 = 180\\
&110 + \text{angle} \ 3 = 180 && 130 + \text{angle} \ 3 = 180\\
&\text{angle} \ 3 = 180 - 110 && \text{angle} \ 3 = 180 - 130 \\
&\text{angle} \ 3 = 70^{\circ} && \text{angle} \ 3 = 50^{\circ}\end{align*}
The angles in the first triangle are \begin{align*}50^{\circ}\end{align*}
Let’s move on to the next pair.
This time we know side lengths, not angles. We need to check whether each set of corresponding sides is proportional. First, let’s write out the pairs of proportional corresponding sides
\begin{align*}\frac{6}{3}, \frac{6}{3}, \frac{4}{1}\end{align*}
The proportions show side lengths from the large triangle on the top and its corresponding side in the small triangle on the bottom. The pairs of sides must have the same proportion in order for the triangles to be similar. We can test whether the three proportions above are the same by dividing each. If the quotient is the same, the pairs of sides must exist in the same proportion to each other.
\begin{align*}\frac{6}{3} = 2\\
\frac{6}{3} = 2\\
\frac{4}{1} = 4\end{align*}
When we divide, only two pairs of sides have the same proportion (2). The third pair of sides does not exist in the same proportion as the other two, so these triangles cannot be similar.
That leaves the last pair. We have been given the measures of some of the angles. If all of the corresponding angles are congruent, then these two figures are similar. We know the measure of three angles in each figure. In fact, they are all corresponding angles. Therefore the one unknown angle in the first figure corresponds to the unknown angle in the second figure.
As we know, the four angles in a quadrilateral must have a sum of \begin{align*}360^{\circ}\end{align*}
Now let’s use what we have learned to solve the problem in the introduction.
Real Life Example Completed
The Mathematical Floor
Here is the original problem once again. Reread it and then answer the questions at the end of this passage.
Mrs. Gilman brought a small group of students over to look at this tile floor in the hallway of the art museum.
“You see, there is even math in the floor,” she said, smiling. Mrs. Gilman is one of those teachers who loves to point out every place where math can be found.
“Okay, I get it,” Jesse started. “I see the squares.”
“There is a lot more math than just squares,” Mrs. Gilman said, walking away with a huge smile on her face.
“She frustrates me sometimes,” Kara whispered, staring at the floor. “Where is the math besides the squares?”
“I think she is talking about the size of the squares,” Hannah chimed in. “See? There are two different sizes.”
“Actually there are three different sizes, and there could be more that I haven’t found yet,” Jesse said.
“Remember when we learned about comparing shapes that are alike and aren’t alike? It has to do with proportions or something like that,” Hannah chimed in again.
All three students stopped talking and began looking at the floor again.
“Oh yeah, congruent and similar figures, but which are which?” Kara asked.
The students are working on which figures in the floor pattern are congruent and which ones are similar.
The congruent figures are exactly the same. We can say that the small dark brown squares are congruent because they are just like each other. They have the same side lengths. What is one other pair of congruent squares?
The similar figures compare squares of different sizes. You can see that the figures are squares, so they all have 90 degree angles. The side lengths are different, but because the angles are congruent, we can say that they have the same shape, but not the same size. This makes them similar figures.
The small dark brown square is similar to the large dark brown square. The small dark brown square is also similar to the square created by the ivory colored tile. There is a relationship between the different squares. Are there any more comparisons? Make a few notes in your notebook.
Vocabulary
- Congruent
- having exactly the same shape and size. All side lengths and angle measures are the same.
- Similar
- having the same shape but not the same size. All angle measures are the same, but side lengths are not.
Technology Integration
Khan Academy Congruent and Similar Triangles
James Sousa, Congruent and Similar Triangles
Time to Practice
Directions: Tell whether the pairs of figures below are congruent, similar, or neither.
Directions: Name the corresponding parts to those given below.
7. \begin{align*}\angle R\end{align*}
8. \begin{align*}MN\end{align*}
9. \begin{align*}\angle O\end{align*}
Directions: Use the relationships between congruent figures to find the measure of \begin{align*}g\end{align*}
10.
Directions: Use the relationships between congruent figures to find the measure of \begin{align*}\angle T\end{align*}
11.
Directions: Answer each of the following questions.
12. Triangles \begin{align*}ABC\end{align*}
13. True or false. If triangles \begin{align*}DEF\end{align*}
14. True or false. Similar figures have exactly the same size and shape.
15. True or false. Congruent figures are exactly the same in every way.
16. Triangles \begin{align*}LMN\end{align*}
17. What is a proportion?
18. True or false. To figure out if two figures are similar, see if their side lengths form a proportion.
19. Define similar figures
20. Define congruent figures.
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