8.5 Proving Triangles are Similar

1. 8.5 Proving Triangles are Similar

2. Objectives/Assignment • Use similarity theorems to prove that two triangles are similar • Assignment: 2-28 even

3. Using Similarity Theorems • In this lesson, you will study 2 alternate ways of proving that two triangles are similar: • Side-Side-Side Similarity Theorem • Side-Angle-Side Similarity Theorem.

4. Side Side Side(SSS) Similarity Theorem • If the corresponding sides of two triangles are proportional, then the triangles are similar. THEN ∆ABC ~ ∆PQR AB BC CA = = PQ QR RP

5. Side Angle Side Similarity Thm. • If an angle of one triangle is congruent to an angle of a second triangle and the lengths of the sides including these angles are proportional, then the triangles are similar. ZX XY If X  M and = PM MN THEN ∆XYZ ~ ∆MNP

6. Ex. 2: Using the SSS Similarity Thm. • Which of the three triangles are similar? To decide which, if any, of the triangles are similar, you need to consider the ratios of the lengths of corresponding sides. Ratios of Side Lengths of ∆ABC and ∆DEF. AB 6 3 CA 12 3 BC 9 3 = = = = = = DE 4 2 FD 8 2 EF 6 2 Because all of the ratios are equal, ∆ABC ~ ∆DEF.

7. Ratios of Side Lengths of ∆ABC ~ ∆GHJ AB 6 CA 12 6 BC 9 = = 1 = = = GH 6 JG 14 7 HJ 10 Because the ratios are not equal, ∆ABC and ∆GHJ are not similar. Since ∆ABC is similar to ∆DEF and ∆ABC is not similar to ∆GHJ, ∆DEF is not similar to ∆GHJ.

8. Ex. 3: Using the SAS Similarity Thm. • Use the given lengths to prove that ∆RST ~ ∆PSQ. Given: SP=4, PR = 12, SQ = 5, and QT = 15; Prove: ∆RST ~ ∆PSQ Use the SAS Similarity Theorem. Begin by finding the ratios of the lengths of the corresponding sides. SR SP + PR 4 + 12 16 = 4 = = = SP SP 4 4

9. ST SQ + QT 5 + 15 20 = 4 = = = SQ SQ 5 5 So, the side lengths SR and ST are proportional to the corresponding side lengths of ∆PSQ. Because S is the included angle in both triangles, use the SAS Similarity Theorem to conclude that ∆RST ~ ∆PSQ.

10. Using Similar Triangles in Real Life • Ex. 6 – Finding Distance indirectly. • To measure the width of a river, you use a surveying technique, as shown in the diagram.

11. Solution By the AA Similarity Postulate, ∆PQR ~ ∆STR. RQ PQ = Write the proportion. RT ST RQ 63 = Substitute. 12 9 RQ = 12 ● 7 Multiply each side by 12. RQ = 84 Solve for TS. So the river is 84 feet wide.