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SSS Similarity

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SSS Triangle Similarity

How can you use the SSS similarity criterion to show that the triangles below are similar?

Watch This

Watch the first half of this video that focuses on SSS:

https://www.youtube.com/watch?v=X6PVWlJn8C0 James Sousa: Triangle Similarity

Guidance

If two triangles are similar it means that all corresponding angle pairs are congruent and all corresponding sides are proportional. However, in order to be sure that two triangles are similar, you do not necessarily need to have information about all sides and all angles.

The SSS criterion for triangle similarity states that if three sides of one triangle are proportional to three sides of another triangle, then the triangles are similar.

In the examples, you will use similarity transformations and criteria for triangle congruence to show why SSS is a criterion for triangle similarity.

Example A

Consider the triangles below with \frac{AB}{FE}=\frac{CA}{DF}=\frac{CB}{DE}=k . Dilate \Delta DEF with a scale factor of k to create \Delta D^\prime E^\prime F^\prime . What do you know about the sides of \Delta D^\prime E^\prime F^\prime ? How do they relate to the sides of \Delta ABC ?

Solution: Below, \Delta DEF is dilated about point  P with a scale factor of k to create \Delta D^\prime E^\prime F^\prime .

The scale factor is k , which is equal to \frac{AB}{FE} and \frac{CA}{DF} and \frac{CB}{DE} . This means:

  • D^\prime F^\prime=k\cdot DF=\frac{CA}{DF}\cdot DF=CA . Therefore, \overline{D^\prime F^\prime} \cong \overline{CA} .
  • D^\prime E^\prime=k\cdot DE=\frac{CB}{DE}\cdot DE=CB . Therefore, \overline{D^\prime E^\prime} \cong \overline{CB} .
  • F^\prime E^\prime=k\cdot FE=\frac{AB}{FE}\cdot FE=AB . Therefore, \overline{F^\prime E^\prime} \cong \overline{AB} .

Example B

Use your work from Example A to prove that \Delta ABC\thicksim\Delta FED .

Solution: From Example A, you know that \overline{D^\prime F^\prime}\cong \overline{CA} , \overline{D^\prime E^\prime}\cong \overline{CB} and \overline{F^\prime E^\prime}\cong \overline{AB} . This means \Delta ABC \cong \Delta F^\prime E^\prime D^\prime by SSS\cong .

Therefore, there must exist a sequence of rigid transformations that will carry \Delta ABC to \Delta F^\prime E^\prime D^\prime .

\Delta ABC\thicksim\Delta FED because a series of rigid transformations will carry \Delta ABC to \Delta F^\prime E^\prime D^\prime , and then a dilation will carry \Delta F^\prime E^\prime D^\prime to \Delta ABC .

All that was known about the original two triangles in Example A was three pairs of proportional sides . You have proved that SSS is a criterion for triangle similarity.

Example C

You want to show that the triangles below are similar by SSS\thicksim . What additional information do you need?

Solution: You have three side lengths for \Delta ABC with \frac{AB}{FE}=\frac{AC}{FD}=3 . Since CB=6 , you need to know that  DE=2 (so that \frac{CB}{DE}=3 ) in order to show that the triangles are similar by SSS\thicksim .

Concept Problem Revisited

Only two pairs of sides and a pair of non-included angles are given. You can't say these triangles are similar by SSA because that is not a criterion for triangle similarity. However, because these are right triangles, you know that the third side of each triangle can be found with the Pythagorean Theorem.

  • For the smaller triangle: 12^2+x^2=15^2\rightarrow x=9 .
  • For the larger triangle: 36^2+x^2=45^2\rightarrow x=27 .

All three pairs of sides are proportional with a ratio of 3. Therefore, the triangles are similar due to SSS\thicksim . See the practice exercises to generalize this proof.

Vocabulary

Rigid transformations are transformations that preserve distance and angles. The rigid transformations are reflections, rotations, and translations.

Two figures are congruent if a sequence of rigid transformations will carry one figure to the other. Congruent figures will always have corresponding angles and sides that are congruent as well.

A similarity transformation is one or more rigid transformations followed by a dilation.

A dilation is an example of a transformation that moves each point along a ray through the point emanating from a fixed center point P , multiplying the distance from the center point by a common scale factor,   k .

Two figures are similar if a similarity transformation will carry one figure to the other. Similar figures will always have corresponding angles congruent and corresponding sides proportional.

SSS , or Side-Side-Side , is a criterion for triangle similarity. The SSS criterion for triangle similarity states that if three sides of one triangle are proportional to three sides of another triangle, then the triangles are similar.

Guided Practice

Are the triangles similar? Explain.

1.

2.

3. You want to show that the triangles below are similar by SSS\thicksim . What additional information do you need?

Answers:

1. All three sides of each triangle are the same. The ratio between each pair of sides is \frac{4x}{x}=4 . Because three pairs of sides are proportional, the triangles are similar by SSS\thicksim .

2. Match the longest side with the longest side and the shortest side with the shortest side. Check all three ratios:

  • \frac{DF}{AC}=\frac{18}{6}=3
  • \frac{DE}{AB}=\frac{15}{5}=3
  • \frac{EF}{BC}=\frac{6}{4}=1.5

Because the three pairs of sides are not proportional, the triangles are not similar.

3. Look for two pairs of sides with lengths in the same ratio.

  • \frac{DE}{AB}=\frac{25}{15}=\frac{5}{3}
  • \frac{DF}{AC}=\frac{35}{21}=\frac{5}{3}

The common ratio is \frac{5}{3} . This means, \frac{EF}{BC} must equal \frac{5}{3} as well.

\frac{15}{BC}&=\frac{5}{3}\\45 &=5BC\\BC &=9

For the triangles to be similar by SSS\thicksim , you need to know that BC=9 .

Practice

1. What does SSS stand for? What does it have to do with similar triangles?

2. Draw an example of two triangles that must be similar due to SSS.

For each pair of triangles below state if they are similar, congruent, or if there is not enough information to determine whether or not they are congruent. If they are similar or congruent, write a similarity or congruence statement.

3.

4.

5.

6.

7.

8. What does the previous problem tell you about equilateral triangles?

9. How are the perimeters of two similar triangles related?

10. One triangle has side lengths of 6, 8, and 10. A similar triangle has a perimeter of 60. What are the lengths of the sides of the similar triangle?

11. One triangle has side lengths of 7, 8, and 14. A similar triangle has a perimeter of 87. What is the ratio between corresponding sides?

12. One triangle has side lengths of 2, 4, and 4. A similar triangle has a perimeter of 30. What are the lengths of the sides of the similar triangle?

13. Find the length of the unmarked side of each triangle in terms of c , b , and k .

14. Use your work from #13 to prove that the two triangles in #13 are similar. What does this tell you about one method for proving that right triangles are similar?

15. Show how the SSS criterion for triangle similarity works: use transformations to help explain why the triangles below are similar. Hint: See Examples A and B for help.

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