The Third Angle Theorem states if two angles are congruent to two angles in another triangle, the third angles are congruent too. Because a triangle has 180∘, the third angle in any triangle is 180∘ minus the other two angle measures. Let’s investigate what happens when two different triangles have the same angle measures.
Investigation: Constructing Similar Triangles
Tools Needed: pencil, paper, protractor, ruler
- Draw a 45∘ angle. Extend the horizontal side and then draw a 60∘ angle on the other side of this side. Extend the other side of the 45∘ angle and the 60∘ angle so that they intersect to form a triangle. What is the measure of the third angle? Measure the length of each side.
- Repeat Step 1 and make the horizontal side between the 45∘ and 60∘ angle at least 1 inch longer than in Step 1. This will make the entire triangle larger. Find the measure of the third angle and measure the length of each side. Find the ratio of the sides. Put the sides opposite the 45∘ angles over each other, the sides opposite the 60∘ angles over each other, and the sides opposite the third angles over each other. What happens?
Watch this video beginning at the 2:09 mark.
AA Similarity Postulate: If two angles in one triangle are congruent to two angles in another triangle, the two triangles are similar.
The AA Similarity Postulate is a shortcut for showing that two triangles are similar. If you know that two angles in one triangle are congruent to two angles in another, which is now enough information to show that the two triangles are similar. Then, you can use the similarity to find the lengths of the sides.
Determining if Two Triangles are Similar
1. Determine if the following two triangles are similar. If so, write the similarity statement.
Find the measure of the third angle in each triangle. m∠G=48∘ and m∠M=30∘ by the Triangle Sum Theorem. Therefore, all three angles are congruent, so the two triangles are similar. △FEG∼△MLN.
2. Determine if the following two triangles are similar. If so, write the similarity statement.
m∠C=39∘ and m∠F=59∘. The angles are not equal, △ABC and △DEF are not similar.
3. Are the following triangles similar? If so, write the similarity statement.
Because AE¯¯¯¯¯¯¯¯ || CD¯¯¯¯¯¯¯¯,∠A≅∠D and ∠C≅∠E by the Alternate Interior Angles Theorem. Therefore, by the AA Similarity Postulate, △ABE∼△DBC.
Are the following triangles similar? If so, write a similarity statement.
No, though ∠MNQ≅∠ONP because they are vertical angles, we need to have two pairs of congruent angles in order to be able to say that the triangles are similar.
Use the diagram to complete each statement.
Answer questions 6-9 about trapezoid ABCD.
Name two similar triangles. How do you know they are similar?
Write a true proportion.
Name two other triangles that might not be similar.
If AB=10,AE=7, and DC=22, find AC. Be careful!
Writing How many angles need to be congruent to show that two triangles are similar? Why?
Writing How do congruent triangles and similar triangles differ? How are they the same?
Use the triangles below for questions 12-15.
AB=20,DE=15, and BC=k.
Are the two triangles similar? How do you know?
Write an expression for FE in terms of k.
If FE=12, what is k?
Fill in the blanks: If an acute angle of a _______ triangle is congruent to an acute angle in another ________ triangle, then the two triangles are _______.
Use the diagram below to answer questions 16-20.
Draw the three separate triangles in the diagram.
Explain why △GDE∼△DFE∼△GFD.
Complete the following proportionality statements.
To view the Review answers, open this PDF file and look for section 7.4.