1. 7.5 Proportions & Similar Triangles
Geometry

2. Objectives/DFA/HW Objective: DFA: p.475 #12
You will use the side splitter & triangle angle bisector theorems to solve problems.
DFA: p.475 #12
HW – pp (2-36 even)

3. Theorem 7.4 Triangle Proportionality Theorem (Side-splitter Theorem)
If a line parallel to one side of a triangle intersects the other two sides, then it divides the two sides proportionally.
If TU ║ QS, then RT RU = TQ US

4. Theorem 7.5 Converse of the Triangle Proportionality Theorem(Side Splitter Theorem)
If a line divides two sides of a triangle proportionally, then it is parallel to the third side. RT RU If
, then TU ║ QS. = TQ US

5. Ex. 1: Finding the length of a segment
In the diagram AB ║ ED, BD = 8, DC = 4, and AE = 12. What is the length of EC?

6. Ex. 2: Determining Parallels
Given the diagram, determine whether MN ║ GH.

7. Theorem 7.6
If three parallel lines intersect two transversals, then they divide the transversals proportionally.
If r ║ s and s║ t and l and m intersect, r, s, and t, then UW VX = WY XZ

8. Theorem 7.5
If a ray bisects an angle of a triangle, then it divides the opposite side into segments whose lengths are proportional to the lengths of the other two sides.
If CD bisects ACB, then AD CA = DB CB

9. Ex. 3: Using Proportionality Theorems
In the diagram 1  2  3, and PQ = 9, QR = 15, and ST = 11. What is the length of TU?

10. Ex. 4: Using the Proportionality Theorem
In the diagram, CAD  DAB. Use the given side lengths to find the length of DC.

11. Solution:
Since AD is an angle bisector of CAB, you can apply Theorem Let x = DC. Then BD = 14 – x. AB BD =
Apply Thm. 7.5 AC DC 9
14-X = 15 X

12. Ex. 4 Continued . . . 9 ● x = 15 (14 – x) Cross product property
Distributive Property
Add 15x to each side
Divide each side by 24.
So, the length of DC is 8.75 units.

13. Use proportionality Theorems in Real Life
Example 5: Finding the length of a segment
Building Construction: You are insulating your attic, as shown. The vertical 2 x 4 studs are evenly spaced. Explain why the diagonal cuts at the tops of the strips of insulation should have the same length.

14. Use proportionality Theorems in Real Life
Because the studs AD, BE and CF are each vertical, you know they are parallel to each other. Using Theorem 8.6, you can conclude that DE AB = EF BC
Because the studs are evenly spaced, you know that DE = EF. So you can conclude that AB = BC, which means that the diagonal cuts at the tops of the strips have the same lengths.

15. Ex. 6: Finding Segment Lengths
In the diagram KL ║ MN. Find the values of the variables.

16. Solution To find the value of x, you can set up a proportion.9 x =
Write the proportion
Cross product property
Distributive property
Add 13.5x to each side.
Divide each side by 22.5
13.5 x
13.5(37.5 – x) = 9x
– 13.5x = 9x
= 22.5 x
22.5 = x
Since KL ║MN, ∆JKL ~ ∆JMN and JK KL = JM MN

17. Solution To find the value of y, you can set up a proportion.9
7.5 =
Write the proportion
Cross product property
Divide each side by 9. y
9y = 7.5(22.5)
y = 18.75