Students classify a triangle with two 55° angles, determine which triangle type is not possible, and classify a 90°-45°-45° triangle by both sides and angles. Part B has five fill-in-the-blank problems using the angle sum to find a missing angle, classifying a 40°-60°-80° triangle, and dual-classifying a 7-7-10 triangle.
Using the 180° angle sum rule to find missing angles — then classifying from those angles — is the highest-order triangle reasoning skill at Grade 5.
Style:
Classifying 2D Shapes
Part A: Multiple Choice
Circle the best answer for each question.
1. A triangle has two angles of 55° each. Classify it by both sides and angles.
A) Scalene acute
B) Isosceles obtuse
C) Isosceles acute
D) Equilateral acute
2. Which of the following is NOT possible?
A) A right scalene triangle
B) An obtuse equilateral triangle
C) An acute isosceles triangle
D) A right isosceles triangle
3. A triangle has angles of 90°, 45°, and 45°. Which classification best describes it?
A) Scalene right
B) Equilateral acute
C) Isosceles right
D) Isosceles obtuse
4. Triangle PQR has sides 6 cm, 6 cm, and 6 cm. What is the measure of each angle?
A) 90°
B) 45°
C) 60°
D) 120°
Part B: Fill in the Blank
Write the correct answer on each line.
1. If two angles of a triangle are 70° each, the third angle is 40 degrees.
2. A triangle with angles 40°, 60°, and 80° is classified as acute by its angles.
3. A triangle with sides 7 cm, 7 cm, and 10 cm is isosceles by its sides and obtuse by its angles.
4. The largest angle in a right triangle always measures exactly 90 degrees.
5. If one angle of a triangle is 90° and another is 35°, the third angle is 55 degrees.
Classifying 2D Shapes
★ Part A: Multiple Choice
Circle the best answer for each question.
1. A triangle has two angles of 55° each. Classify it by both sides and angles.
A) Scalene acute
B) Isosceles obtuse
C) Isosceles acute
D) Equilateral acute
2. Which of the following is NOT possible?
A) A right scalene triangle
B) An obtuse equilateral triangle
C) An acute isosceles triangle
D) A right isosceles triangle
3. A triangle has angles of 90°, 45°, and 45°. Which classification best describes it?
A) Scalene right
B) Equilateral acute
C) Isosceles right
D) Isosceles obtuse
4. Triangle PQR has sides 6 cm, 6 cm, and 6 cm. What is the measure of each angle?
A) 90°
B) 45°
C) 60°
D) 120°
★ Part B: Fill in the Blank
Write the correct answer on each line.
1) If two angles of a triangle are 70° each, the third angle is 40 degrees.
2) A triangle with angles 40°, 60°, and 80° is classified as acute by its angles.
3) A triangle with sides 7 cm, 7 cm, and 10 cm is isosceles by its sides and obtuse by its angles.
4) The largest angle in a right triangle always measures exactly 90 degrees.
5) If one angle of a triangle is 90° and another is 35°, the third angle is 55 degrees.
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9 Questions
12-18 minutes
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