What if you were given a pair of triangles and the side lengths for all three of their sides? How could you use this information to determine if the two triangles are similar? After completing this Concept, you'll be able to use the SSS Similarity Theorem to decide if two triangles are similar.
Watch This
CK12 Foundation: Chapter7SSSSimilarityA
Watch this video beginning at the 2:09 mark.
James Sousa: Similar Triangles
Watch the first part of this video.
James Sousa: Similar Triangles Using SSS and SAS
Guidance
If you do not know any angle measures, can you say two triangles are similar? Let’s investigate this to see.
Investigation: SSS Similarity
Tools Needed: ruler, compass, protractor, paper, pencil
 Construct a triangle with sides 6 cm, 8 cm, and 10 cm.
 Construct a second triangle with sides 9 cm, 12 cm, and 15 cm.
 Using your protractor, measure the angles in both triangles. What do you notice?
 Line up the corresponding sides. Write down the ratios of these sides. What happens?
To see an animated construction of this, click: http://www.mathsisfun.com/geometry/constructrulercompass1.html
From #3, you should notice that the angles in the two triangles are equal. Second, when the corresponding sides are lined up, the sides are all in the same proportion,
SSS Similarity Theorem: If the corresponding sides of two triangles are proportional, then the two triangles are similar.
Example A
Determine if the following triangles are similar. If so, explain why and write the similarity statement.
We will need to find the ratios for the corresponding sides of the triangles and see if they are all the same. Start with the longest sides and work down to the shortest sides.
Since all the ratios are the same,
Example B
Find
According to the similarity statement, the corresponding sides are:
Example C
Determine if the following triangles are similar. If so, explain why and write the similarity statement.
We will need to find the ratios for the corresponding sides of the triangles and see if they are all the same. Start with the longest sides and work down to the shortest sides.
Since the ratios are not all the same, the triangles are not similar.
Watch this video for help with the Examples above.
CK12 Foundation: Chapter7SSSSimilarityB
Vocabulary
Two triangles are similar if all their corresponding angles are congruent (exactly the same) and their corresponding sides are proportional (in the same ratio).
Guided Practice
Determine if any of the triangles below are similar. Compare two triangles at a time.
1. Is
2. Is
3. Is
Answers:
1.
Reduce each fraction to see if they are equal.
2.
3.
Practice
Fill in the blanks.
 If all three sides in one triangle are __________________ to the three sides in another, then the two triangles are similar.
 Two triangles are similar if the corresponding sides are _____________.
Use the following diagram for questions 35. The diagram is to scale.
 Are the two triangles similar? Explain your answer.
 Are the two triangles congruent? Explain your answer.
 What is the scale factor for the two triangles?
Fill in the blanks in the statements below. Use the diagram to the left.

△ABC∼△ _____ 
AB?=BC?=AC?  If
△ABC had an altitude,AG=10 , what would be the length of altitudeDH¯¯¯¯¯¯¯¯¯ ?  Find the perimeter of
△ABC and△DEF . Find the ratio of the perimeters.
Use the diagram to the right for questions 1015.

△ABC∼△ _____  Why are the two triangles similar?
 Find
ED . 
BD?=?BC=DE?  Is
ADDB=CEEB true?  Is
ADDB=ACDE true?
Find the value of the missing variable(s) that makes the two triangles similar.