1. Similar Right Triangles
Chapter 8

2. Activity: Investigating similar right triangles. Do in pairs or threes
Cut an index card along one of its diagonals.
On one of the right triangles, draw an altitude from the right angle to the hypotenuse. Cut along the altitude to form two right triangles.
You should now have three right triangles. Compare the triangles. What special property do they share? Explain.

3. Theorem 8.1 (page 518)
If the altitude is drawn to the hypotenuse of a right triangle, then the two triangles formed are similar to the original triangle and to each other.
∆CBD ~ ∆ABC, ∆ACD ~ ∆ABC, ∆CBD ~ ∆ACD

4. Proportions in right triangles
In chapter 7, you learned that two triangles are similar if two of their corresponding angles are congruent. For example ∆PQR ~ ∆STU. Recall that the corresponding side lengths of similar triangles are in proportion.

5. Using a geometric mean to solve problemsD
In right ∆ABC, altitude CD is drawn to the hypotenuse, forming two smaller right triangles that are similar to ∆ABC From Theorem 8.1, you know that ∆CBD~∆ACD~∆ABC. A C B D
They ALL look the same!
Similar = same shape, different size

6. Geometric Mean Theorems
Theorem 8.2: In a right triangle, the altitude from the right angle to the hypotenuse divides the hypotenuse into two segments. The length of the altitude is the geometric mean of the lengths of the two segments
Theorem 8.3: In a right triangle, the altitude from the right angle to the hypotenuse divides the hypotenuse into two segments. The length of each leg of the right triangle is the geometric mean of the lengths of the hypotenuse and the segment of the hypotenuse that is adjacent to the leg.

7. Geometric Mean Theorems
If you want to find the Altitude… use Geometric Mean BD CD = GM CD AD
CD is the Geometric Mean
of AD
and BD

8. Example: Use Geometric Mean to find the Altitude of the Triangle6 x = x 3
18 = x2
√18 = x
√9 ∙ √2 = x
3 √2 = x

9. Geometric Mean Theorems
If you want to find the side on the right side of the triangle… use Geometric Mean GM AB CB = B A CB DB
CB is the Geometric Mean
of DB
and AB

10. Example: Find y (the right leg of the triangle) using Geometric Mean
Rotate the triangle to help visualize what part you need
Y is the leg on the right side
5 + 2 y = y 2 7 y = y 2
14 = y2
√14 = y

11. Geometric Mean Theorems
If you want to find the side on the LEFT side of the triangle… use Geometric Mean GM AB AC = B A AC AD
AC is the Geometric Mean
of AD
and AB

12. Example: Find y (the left leg of the triangle) using Geometric Mean35 35 y = y 7 7
245 = y2 y
Y is the leg on the Left side
√245 = y
7√5 = y

13. Ex. 1: Finding the Height of a Roof
Roof Height. A roof has a cross section that is a right angle. The diagram shows the approximate dimensions of this cross section.
A. Identify the similar triangles.
B. Find the height h of the roof.

14. Solution:
You may find it helpful to sketch the three similar triangles so that the corresponding angles and sides have the same orientation. Mark the congruent angles. Notice that some sides appear in more than one triangle. For instance XY is the hypotenuse in ∆XYW and the shorter leg in ∆XZY.
∆XYW ~ ∆YZW ~ ∆XZY.

15. Solution for b. Use the fact that ∆XYW ~ ∆XZY to write a proportion.
Corresponding side lengths are in proportion. = ZY XZ h
3.1 =
Substitute values.
5.5
6.3
6.3h = 5.5(3.1)
Cross Product property
Solve for unknown h.
h ≈ 2.7
The height of the roof is about 2.7 meters.