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4.1B Triangle Properties
Teacher Candidate: Mary Takle
Lesson Date: 1/3/2011
Cooperating Teacher: Dawn Withee
Grade: 9-11 (mostly grade 10 students)
School District: Edmonds School District
School: Lynnwood High School
University Supervisor: Andrea Escame-Hedger
Subject/Grade Level: Geometry
Unit: Triangle Properties
Grouping for Instruction: Students will sit in heterogeneous groups of 3-4 and work cooperatively using the
Complex-Instruction model.
Objective(s)/Learning Target(s): Students will identify and classify triangles and explore triangle
properties
Standard(s): 3.A Know, explain, and apply basic postulates and theorems about triangles and the special lines,
line segments, and rays associated with a triangle.
Prior Learning: We explored types of triangles before winter break – now we will come up with the formal
definitions for triangles.
Homework: McDougal Little Geometry: pg 221: 14-16, 21-29
Materials/Preparation Needed: Task Card, Geometer’s Sketchpad Files, Pencil, Paper, Triangle Cards, small
whiteboards & markers
Assessment of learning: Formative: Discussion with groups as I circulate through the class during group
work.
Instructional Plan and Strategies
Opener: No homework to check.
Vocab Packet & Demos (20 Minutes)
Explore: Pass out Task Card (20 Minutes)
Wrap-up: Pass the white board Triangle Game (10 minutes)
Pass the Whiteboard (WB) and the pen.
The group draws the requested figureon the WB and the board is
passed to the next group over to score. Each correct answer is scored 1
point for that group. The RM for neighboring group keeps a group’s
score. The pen moves from one person to the next in the group and the
next card is shown, so each group member is required to draw a figure.
Scalene Triangle, Isosceles, Triangle, Isosceles Right Triangle, Obtuse
Isosceles Triangle, Scalene Right Triangle, Equilateral Triangle,
Equiangular Triangle, Scalene Acute Triangle, Right Triangle,
Equilateral Right Triangle, Scalene Obtuse Triangle, Acute Isosceles
Triangle, Equilateral Obtuse Triangle, Equilateral Acute Triangle
Modifications (Gear Up/Gear Down):
Gear-up - Problem 2 should challenge the groups who get through the first set of problems quickly.
Gear-down - Groups might need support identifying which theorems to use. Vocab can be challenging for ELL
students, so reminding them to refer to the packet should help.
Reflection:
Geometry 4.1 Task – Classifying Triangles
Learning Objective: You will classify a triangle by the lengths of its sides and the measures of
its angles. You will apply the Triangle Sum Theorem and Exterior Angle Theorem.
Problem 1: In each figure, solve for the unknown variable(s). (Show your work!) Then,
classify the triangle based on angles and sides.
(2𝑥 − 2)°
24°
𝑥°
45°
(2𝑥 + 18)°
(3𝑥 + 6)°
25°
𝑦°
𝑥°
20°
𝑥°
43°
𝑦°
𝑦°
75°
𝑥°
64°
𝑥°
Geometry 4.1 Task – Classifying Triangles
Learning Objective: You will classify a triangle by the lengths of its sides and the measures of
its angles. You will apply the Triangle Sum Theorem and Exterior Angle Theorem.
Problem 2: Find the measure of each of the lettered angles in the diagram below without using
a protractor.
t
m = ______
n = ______
p = ______
q = ______
r = ______
s = ______
t = _______
u = ______
****Teacher Check Point****
Geometry 4.1 Task – Classifying Triangles
Vocab & Demos
Triangle
Types of Triangles
sides Scalene
A polygon with 3 sides
No congruent sides
Isosceles
2 congruent sides
Equilateral
3 congruent sides
angles Acute
Obtuse
Right
Equiangular
Interior Angles
3 angles < 90°
1 angle >90°
1 angle = 90°
3 congruent angles = 60°
The angles inside the
polygon
Exterior Angles
The angles that form
linear pairs with the
interior angles of a
polygon
Triangle Sum
Theorem (4.1)
The sum of the measures
of the interior angles of
a triangle is 180°
mABC = 36°
mCAB = 125°
mBCA = 19°
A
..\Chapter 4\triangle
sum.gsp
B
mABC + mCAB + mBCA = 180°
C
Geometry 4.1 Task – Classifying Triangles
Exterior Angle
Theorem (4.2)
The measure of an
exterior angle of a
triangle is equal to the
sum of the measures of
the two non-adjacent
interior angles.
mABC = 76.0°
mBAC = 58.4°
mABC + mBAC = 134.4°
A
mACD = 134.4°
..\Chapter 4\exterior
angle theorem.gsp
B
D C
Corollary to Triangle The acute angles of a
Sum Theorem
right triangle are
complementary
mABC = 39°
B
mABC + mBCA = 90°
mBCA = 51°
..\Chapter 4\right
triangle.gsp
A
C
Daily Plan Sheet
Subject: Geometry
Date: 1/3/2011
Lesson: Triangle Properties
Goal(s)/Standard(s):
 You will classify a triangle by the lengths of its sides and the
measures of its angles. You will apply the Triangle Sum Theorem
and Exterior Angle Theorem.
Look fors:




Everyone working together
Showing your work
Group discussion
Use of math vocabulary – scalene, isosceles, equilateral, acute,
obtuse, right, equiangular, interior, exterior
Today you will need someone who can…







Write equations
Solve equations
Use theorems
Analyze
Understand vocabulary
Ask “Why?”
Say “Wait, I don’t understand!”
Materials:
 Vocabulary sheets, task card, pencil, whiteboard, markers
Pass the Whiteboard and the Pen Rules
1. Only the designated person can use the whiteboard marker. The other group
members are permitted to talk, but not draw.
2. No pencils/pens on the table. You may use your vocab sheets.
3. Your team has one minute to come up with the drawing.
4. At the end of one minute, your white board must be passed to the group that is
designated to score your answer.
5. The Resource Manager keeps track of the score.
6. Each correct answer is worth 1 point. No partial credit.
7. For each new problem, the whiteboard and pen rotate to the next group
member. Task – Communication – Resource – Norm.
8. No sharing answers outside your group.
9. Teams caught cheating will lose one point.
Rotation pattern
8
7
6
4
3
2
5
1
Scalene Triangle
Isosceles Triangle
Isosceles Right
Triangle
Obtuse Isosceles
Triangle
Scalene Right
Triangle
Equilateral Triangle
Equiangular Triangle
Scalene Acute
Triangle
Right Triangle
Equilateral Right
Triangle
Scalene Obtuse
Triangle
Acute Isosceles
Triangle
Equilateral Obtuse
Triangle
Equilateral Acute
Triangle
Equiangular Scalene
Triangle
Equiangular
Isosceles Triangle
Exterior angle of a
triangle
Interior angle of a
triangle