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REACHING ALL LEARNERS
Alternative Explore
Materials: tracing paper, scissors
Have students to compare the angles of each triangle in
Explore in other ways (folding, tracing, cutting) and sort them.
Common Misconceptions
➤ Students classify a triangle with 2 acute angles as an
acute triangle.
How to Help: Show students that a right triangle and an
obtuse triangle also have 2 acute angles. For a triangle to be
acute, all 3 angles must be acute.
Sample Answers
Each triangle is acute because
all the angles in each triangle
are less than 90°.
1. a)
b)
Each triangle contains one
angle greater than 90°, so
each triangle is obtuse.
c)
Each triangle contains one
angle equal to 90°, so each
triangle is right.
4. Each triangle has one side that is 2 cm long,
one side that is 2.8 cm long, and exactly
2 acute angles, so does this triangle:
AFTER
Right
J 35°, K 55°, L 90°
4.7 cm
Acute
M 59°, N 85°, P 36°
Obtuse
D 19°, E 45°, F 116°
2.8 cm
2 cm
Connect
Invite students to share the strategies they used
to sort the triangles. They can demonstrate
using an overhead projector or have students
gather around a desk or table.
Discuss Connect.
Practice
Geoboards, geobands, and square dot paper
(PM 25) are required for questions 1 and 6.
Protractors are required for question 2.
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Obtuse
A 25°, B 130°, C 25°
Unit 3 • Lesson 3 • Student page 88
Assessment Focus: Question 6
Students should be able to use geoboards and
geobands to create obtuse triangles that are
scalene and isosceles. The equilateral triangle is
impossible but it may take some students some
extra time to figure this out. The angles of an
equilateral triangle are all 60°, acute. Students
should be able to transfer their creations on
their geoboards to the dot paper.
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5. a) An equilateral triangle has all sides equal, a scalene
triangle has no sides equal, and an isosceles triangle has
2 sides equal.
Equilateral
6. a)
Equilateral
Isosceles; Scalene
Scalene
Isosceles
b)
b) An equilateral triangle always has 3 equal angles.
Each angle measures 60°.
7. a)
b)
c)
No
No
Yes
REFLECT: I can sort triangles by angle measures. For example,
2,
3,
4,
5,
6,
4
6
16
25
24
I can put triangles with all acute angles in one group, triangles
with one right angle in another group, and triangles with one
obtuse angle in a third group. I can also sort triangles by the
number of equal angles they have. For example, I can put
triangles with no equal angles in one group, triangles with
2 equal angles in another group, and triangles with 3 equal
angles in a third group.
ASSESSMENT FOR LEARNING
What to Look For
What to Do
Reasoning; Applying concepts
✔ Students understand that triangles can
be sorted and classified by angle
measures as well as by side lengths.
Extra Support: Students can use Step-by-Step 3 (Master 3.20)
to complete question 6.
Communication
✔ Students use appropriate mathematical
terminologies to describe, compare,
and classify triangles.
Extra Practice: Students can do the Additional Activity, String
Triangles (Master 3.15).
Students can complete Extra Practice 2 (Master 3.28).
Extension: Have students use their triangles from Practice
question 1. They measure the angles in each triangle and add
them. They write to explain their findings.
Recording and Reporting
Master 3.2 Ongoing Observations:
Geometry
Unit 3 • Lesson 3 • Student page 89
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