1. 8.6 Proportions & Similar Triangles
Unit IIA Day 10

2. Do Now:
Review these highlights from last time:
Triangle Proportionality Thm.: If a line parallel to one side of a triangle intersects the other two sides, then it divides the two sides proportionally.
Triangle Proportionality Converse: If a line divides two sides of a triangle proportionally, then it is parallel to the third side.
Thm. 8.6: If three parallel lines intersect two transversals, then they divide the transversals proportionally.

3. Discovery…

4. Theorem 8.7
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/DB = AC/BC

5. Ex. 4: Using the Proportionality Theorem
In the diagram, CAD  DAB. Use the given side lengths to find the length of DC.
Since AD is an angle bisector of CAB, you can apply Theorem Let x = DC. Then BD = 14 – x.
AB/AC = BD/DC – thm 8.7
9/15 = (14-x)/x – substitute
9 ● x = 15 (14 – x)
9x = 210 – 15x
24x= 210
x= 8.75

6. Ex. 4A
In the diagram, LKM  MKN. Use the given side lengths to find the length of MN.
12.75

7. Ex. 5: Proportionality Theorems in Real Life
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.
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/EF = AB/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.

8. Ex. 6: Finding Segment Lengths
In the diagram KL || MN. Find the values of the variables.
To find the value of x, you can set up a proportion.
9/13.5 = (37.5 – x)/x
13.5(37.5 – x) = 9x
– 13.5x = 9x
= 22.5 x
22.5 = x
Since KL ║MN, ∆JKL ~ ∆JMN and JK/JM = KL/MN
To find the value of y, you can set up a proportion.
9/(13/5+9) = 7.5/y
9y = 7.5(22.5)
y = 18.75

9. More Practice
Find the value of the variable.
a) c) b)

10. Closure
Explain what you know about a triangle that has a ray bisecting one of the angles. (Draw a picture.)
The ray cuts the third side of the triangle into segments whose lengths are proportional to the lengths of the other two sides.
Homework: p. 502 # odd