1. Day 2 agenda Go over homework- 5 min Take up for effort grade
Warm-up- 15 min
Hands-on activity on properties of midsegments
5.1 notes- 30 min
5.1 practice- 10 min
Computer lab- 30 min
The Geometer’s Sketchpad

2. Warm-Up: Split students into randomly assigned groups of three
Give each group a pre-cut paper triangle and a ruler
Have each group label the three vertices and the midpoints of two sides of the triangle (students should use the ruler to find the midpoints)
Students should then draw a line connecting the two midpoints
Ask each group to then make a conjecture about how the segment joining the midpoints of two sides of their triangle is related to the third side of the triangle
Expected answers: parallel, half the length, shorter

3. Midsegments of Triangles 5.1
Today’s goals By the end of class today, YOU should be able to…
1. Define and use the properties of midsegments to solve problems for unknowns.
2. Use the properties of midsegments to make statements about parallel segments in a given triangle.
3. Understand and write coordinate proofs.

4. Midsegments
A midsegment of a triangle is a segment connecting the midpoints of two sides of a triangle.

5. Triangle Midsegment Theorem
If a segment joins the midpoints of two sides of a triangle, then the segment is parallel to the third side, and is half its length.

6. Proving the Triangle Midsegment Theorem…
To prove the Triangle Midsegement Theorem, use coordinate geometry and algebra. This style of proof is called a coordinate proof. To begin the proof:
Place the triangle in a convenient spot on the coordinate plane.
Choose variables for the coordinates of the vertices.

7. Coordinate proofs cont…
State what you are given.
State what you want to prove.
Use the midpoint formula to find the midpoints of two sides of the triangle.
To prove that the midsegment and third side are parallel, show that their slopes are equal.
To prove that the midsegment is half the length of the third side, use the distance formula to calculate the length of both segments.
*See section 5.1 for a complete version of a coordinate proof for the Triangle Midsegment Theorem

8. Ex.1: Find the lengths
Using the following illustration (where H, J, and K are the midpoints), find the lengths of HJ, JK, and FG:

9. Ex.1: Solution HJ = ½ EG = ½ (100) = 50
Use the Triangle Midsegment Theorem to show that the midsegment = ½ the length of the third side
JK = ½ EF = ½ (60) = 30
HK = ½ FG
FG = 2 HK = 2 (40) = 80

10. You Try…
Use the following triangle to solve the questions below (A, B, & C are midpoints):
1. DF=120, EF=90, BC=40. Find AB, AC, & DE.
2. EF=10x, AB=4x, AC=10, DE=12x. Find BC & AC.

11. Ex.2: Identify the parallel segments
If A, B, & C are midpoints, which segment is parallel to AC?

12. By the Triangle Midsegment Theorem, AC ll EF
Ex.2: Solution
By the Triangle Midsegment Theorem, AC ll EF

13. You Try… If A, B, & C are midpoints: Which segment is parallel to DE?
Which segment is parallel to AB?

14. Practice
Solve with a partner of your choice…
The triangular face of the Rock and Roll Hall of Fame in Cleveland, Ohio is isosceles. The length of the base is 229 ft 6 in. What is the length of a segment located half way up the face of the Rock and Roll Hall of Fame Explain your reasoning.

15. Homework Page 246 #s 2, 4, 6, 7, 13, 30 Page 247 #s 26, 34, 35
The assignment can also be found online at:

16. Additional practice Take the students to the computer lab
Hand out worksheet called Investigation: The Midsegment Theorem and ask the students to complete as much of the sheet as possible during the remaining class time