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Geometry
Teacher Key
Triangles
Classify each triangle by the size of its angles (acute, right, or obtuse) and the length
of its sides (equilateral, isosceles, or scalene).
A.
B.
obtuse, isosceles
acute, equilateral
C.
D.
right, scalene
E.
right, isosceles
F.
acute, isosceles
obtuse, scalene
Given two angles of a triangle, determine the measure of the third angle.
G. 80°, 90°
10°
H. 120°, 20°
40°
I. 15°, 55°
110°
J. 100°, 30°
50°
© 2003 CompassLearning, Inc.
Activity 56264
Geometry
Teacher Key
Triangles
Determine whether the angle or side measurements given can form a triangle. If a
triangle can be formed, classify it by its angles or its side lengths.
Triangles may be classified by their angles or side lengths.
K. 90°, 45°, 65°
Cannot form a triangle
N. 135°, 35°, 5°
Cannot form a triangle
Q. 8 mm, 8 mm, 8 mm
equilateral triangle
© 2003 CompassLearning, Inc.
L. 70°, 40°, 70°
acute triangle
O. 5 in., 3 in., 2 in.
Cannot form a triangle
R. 2 ft., 12 ft., 9 ft.
Cannot form a triangle
M. 100°, 25°, 55°
obtuse triangle
P. 3 cm, 6 cm, 8 cm
scalene triangle
S. 4 m, 3 m, 4 m
isosceles triangle
Activity 56264
Geometry
Teacher Key
Connections
Think About It
Identify the angle classifications (acute, right, or obtuse) and side classifications
(equilateral, isosceles, or scalene) that cannot be combined to form a triangle. Explain why
the combination is not possible.
SAMPLE RESPONSE: It is not possible to form a right equilateral or obtuse
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equilateral triangle. If a triangle has three sides of the same length, it will also have
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three angles of the same measure. Also, a triangle can have only one right angle or
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one obtuse angle.
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Straws and String
In this activity you will explore triangles.
Materials
drinking straws
ruler
scissors
yarn
protractor
pencil
Procedure
Step 1: Cut the drinking straws into different length pieces.
Step 2: Cut a piece of yarn long enough to fit through 3 straw pieces.
(About 30 inches should be long enough.)
Step 3: Choose 3 different pieces of straw. Record the lengths of those pieces in the table
below.
Step 4: Place the yarn through the straws and pull the string to try to form a triangle from
the straw pieces. Record in the table whether the pieces formed a triangle or not.
Step 5: If the straw pieces formed a triangle, use a protractor to measure the angles of the
triangle. Record the angle measurements in the table.
Step 6: Repeat steps 3 – 5 with new pieces of straw.
© 2003 CompassLearning, Inc.
Activity 56264
Geometry
Teacher Key
Connections
Straws and String Table
Student answers will vary.
Straw Lengths
Forms a Triangle?
Angle Measures
ex. 8 in., 8 in., 8 in.
Yes
60°, 60°, 60°
Were you able to form any triangles with different side lengths but the same angle
measures?
Students should find that triangles with different side lengths can have the same
angle measure. These triangles are similar.
© 2003 CompassLearning, Inc.
Activity 56264