Lesson 7.1: Classifying Angles by Size
Comprehensive lesson on Classifying Angles by Size in Grade 5 Mathematics.
⏱️ Reading Time: 20 mins
• ❓ 50 Interactive Practice Questions
🌟
Real-Life Story: Types of Angles
Carpentry Workshop: Framing wooden roof trusses and checking corner squareness with carpenter squares.
💡 Core Mathematical Concepts: Classifying Angles by Size
Focus areas: Acute (<90°), Right (90°), Obtuse (90°-180°), Straight (180°)..
| Concept / Term | Mathematical Meaning | Real-Life Application |
|---|---|---|
| Acute Angle | An angle measuring greater than 0° and less than 90° | Scissors opening, tip of pencil, slice of cake |
| Right Angle | An exact 90° square corner | Window corners, door frames, book edges |
| Obtuse Angle | An angle measuring greater than 90° and less than 180° | Reclining deck chair, open laptop screen |
| Straight Angle | An exact 180° flat angle forming a continuous straight line | Stretched straight ruler |
📝 Step-by-Step Worked Examples
Worked Example 1: Classify 65°
Between 0° and 90° -> Acute angle.
Worked Example 2: Classify 135°
Between 90° and 180° -> Obtuse angle.
⚠️ Common Misconceptions to Avoid
Common Misconception: Confusing arm length with angle size
Correction: An angle's size depends on rotation between rays, NOT how long the lines are drawn.
Common Misconception: Assuming right angles only point upright
Correction: A right angle can be oriented in any diagonal or upside-down position.
🎯
Self-Check Challenge
A street intersection has two roads meeting at an angle of 115°. Classify this angle as acute, right, or obtuse.
Answer & Explanation:
Since 115° is greater than 90° and less than 180°, it is an obtuse angle.
🎯 Key Learning Takeaways
Master core definitions of Classifying Angles by Size.
Apply mathematical principles to everyday scenarios.
Avoid common calculation and conceptual pitfalls.
📄 Printable Worksheets (5 Available)
Download ready-to-print PDFs for offline student practice with separate parent/teacher answer keys.
G5-M-CH07-WS05
A4 • 15 Qs
Types of Angles: Worksheet 5
Practice Assessment 5 • Central Botanical Gardens Context
G5-M-CH07-WS04
A4 • 15 Qs
Types of Angles: Worksheet 4
Practice Assessment 4 • Eco-Friendly Solar Park Context
G5-M-CH07-WS03
A4 • 15 Qs
Types of Angles: Worksheet 3
Practice Assessment 3 • Harbor Marine Terminal Context
G5-M-CH07-WS02
A4 • 15 Qs
Types of Angles: Worksheet 2
Practice Assessment 2 • Science & Discovery Museum Context
G5-M-CH07-WS01
A4 • 15 Qs
Types of Angles: Worksheet 1
Practice Assessment 1 • City Regional Airport Context
✍️ Lesson Practice Questions (50 Questions)
Solve these real-world challenges online, test your understanding, and reveal instant step-by-step explanations.
1
Short Answer
Hard
2 Marks
At what exact time between 1:00 and 2:00 do the hands of a clock form an acute angle: at 1:10 or at 1:50?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
1:10
Step-by-Step Explanation:
At 1:10 the hands are close together (approx 25° acute angle). At 1:50 the hands are widely spread forming an obtuse/reflex angle.
2
Short Answer
Hard
2 Marks
In a right-angled triangle, one acute angle measures 34°. What is the measure of the other acute angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
56°
Step-by-Step Explanation:
The two acute angles in a right triangle add to 90°. 90° - 34° = 56°.
3
Short Answer
Hard
2 Marks
Explain how you can use the corner of an index card (which is 90°) to determine whether an unknown angle is acute or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Place the index card vertex at the angle's corner with one edge aligned. If the other ray hides inside the card, the angle is acute (90°).
Step-by-Step Explanation:
A standard 90° corner serves as a physical benchmark for angle classification.
4
Short Answer
Hard
2 Marks
Can a triangle have two right angles? Explain using the property of angle sums in triangles.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
No. The sum of all three angles in any triangle is always 180°. Two right angles would total 90° + 90° = 180°, leaving 0° for the third angle, which cannot form a closed triangle.
Step-by-Step Explanation:
Two 90° angles equal 180°, making a third angle impossible in a triangle.
5
Short Answer
Medium
1 Mark
A student claims: 'An obtuse angle can measure 195°.' Explain the error and give the correct classification.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Reflex angle
Step-by-Step Explanation:
Obtuse angles must be less than 180°. An angle greater than 180° is a reflex angle.
6
Short Answer
Medium
1 Mark
Jake looks at a 95° angle and calls it an acute angle because it is close to 90°. Why is Jake mistaken?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
It is an obtuse angle
Step-by-Step Explanation:
Any angle strictly greater than 90° (even 91° or 95°) is classified as obtuse, not acute.
7
Fill in the Blanks
Medium
1 Mark
Find the missing angle to complete a right angle: 38° + [ ? ]° = 90°.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
52
Step-by-Step Explanation:
90 - 38 = 52°.
8
Fill in the Blanks
Medium
1 Mark
Identify the angle classification: Angle X = 90°. Angle X is a [ ? ] angle.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
right
Step-by-Step Explanation:
An angle of exactly 90° is called a right angle.
9
Short Answer
Medium
1 Mark
A carpenter joins two wooden beams forming a straight line of 180°. If one angle is 65°, what is the measure of the adjacent supplementary angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
115°
Step-by-Step Explanation:
Angles on a straight line add to 180°. 180° - 65° = 115°.
10
Short Answer
Medium
1 Mark
Look at an analog clock at 5:00. What type of angle (acute, right, or obtuse) is formed between the hour hand and the minute hand?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Obtuse angle
Step-by-Step Explanation:
5 intervals of 30° = 150°. Since 150° is between 90° and 180°, it is an obtuse angle.
11
Short Answer
Medium
1 Mark
At the Central Botanical Gardens, an architect designs an A-frame roof where the apex angle measures 72°. Classify this roof angle as acute, right, or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Acute angle
Step-by-Step Explanation:
Since 72° is strictly less than 90°, it is an acute angle.
12
Multiple Choice
Easy
1 Mark
What is the measure of a straight angle?
A
180°
B
90°
C
360°
D
270°
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
180°
Step-by-Step Explanation:
A straight angle forms a flat straight line and measures exactly 180°.
13
Multiple Choice
Easy
1 Mark
An angle measuring 135° is classified as which type of angle?
A
Obtuse angle
B
Acute angle
C
Right angle
D
Straight angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Obtuse angle
Step-by-Step Explanation:
An angle greater than 90° and less than 180° is an obtuse angle.
14
Multiple Choice
Easy
1 Mark
What type of angle is formed by the hands of a clock at 3:00?
A
Right angle (90°)
B
Acute angle
C
Obtuse angle
D
Straight angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Right angle (90°)
Step-by-Step Explanation:
At 3:00, the minute hand points to 12 and the hour hand to 3, forming a 90° right angle.
15
Multiple Choice
Easy
1 Mark
Which type of angle measures strictly less than 90°?
A
Acute angle
B
Obtuse angle
C
Right angle
D
Reflex angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Acute angle
Step-by-Step Explanation:
An acute angle is strictly between 0° and 90°.
16
Short Answer
Hard
2 Marks
At what exact time between 1:00 and 2:00 do the hands of a clock form an acute angle: at 1:10 or at 1:50?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
1:10
Step-by-Step Explanation:
At 1:10 the hands are close together (approx 25° acute angle). At 1:50 the hands are widely spread forming an obtuse/reflex angle.
17
Short Answer
Hard
2 Marks
In a right-angled triangle, one acute angle measures 34°. What is the measure of the other acute angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
56°
Step-by-Step Explanation:
The two acute angles in a right triangle add to 90°. 90° - 34° = 56°.
18
Short Answer
Hard
2 Marks
Explain how you can use the corner of an index card (which is 90°) to determine whether an unknown angle is acute or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Place the index card vertex at the angle's corner with one edge aligned. If the other ray hides inside the card, the angle is acute (90°).
Step-by-Step Explanation:
A standard 90° corner serves as a physical benchmark for angle classification.
19
Short Answer
Hard
2 Marks
Can a triangle have two right angles? Explain using the property of angle sums in triangles.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
No. The sum of all three angles in any triangle is always 180°. Two right angles would total 90° + 90° = 180°, leaving 0° for the third angle, which cannot form a closed triangle.
Step-by-Step Explanation:
Two 90° angles equal 180°, making a third angle impossible in a triangle.
20
Short Answer
Medium
1 Mark
A student claims: 'An obtuse angle can measure 195°.' Explain the error and give the correct classification.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Reflex angle
Step-by-Step Explanation:
Obtuse angles must be less than 180°. An angle greater than 180° is a reflex angle.
21
Short Answer
Medium
1 Mark
Jake looks at a 95° angle and calls it an acute angle because it is close to 90°. Why is Jake mistaken?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
It is an obtuse angle
Step-by-Step Explanation:
Any angle strictly greater than 90° (even 91° or 95°) is classified as obtuse, not acute.
22
Fill in the Blanks
Medium
1 Mark
Find the missing angle to complete a right angle: 38° + [ ? ]° = 90°.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
52
Step-by-Step Explanation:
90 - 38 = 52°.
23
Fill in the Blanks
Medium
1 Mark
Identify the angle classification: Angle X = 90°. Angle X is a [ ? ] angle.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
right
Step-by-Step Explanation:
An angle of exactly 90° is called a right angle.
24
Short Answer
Medium
1 Mark
A carpenter joins two wooden beams forming a straight line of 180°. If one angle is 65°, what is the measure of the adjacent supplementary angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
115°
Step-by-Step Explanation:
Angles on a straight line add to 180°. 180° - 65° = 115°.
25
Short Answer
Medium
1 Mark
Look at an analog clock at 5:00. What type of angle (acute, right, or obtuse) is formed between the hour hand and the minute hand?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Obtuse angle
Step-by-Step Explanation:
5 intervals of 30° = 150°. Since 150° is between 90° and 180°, it is an obtuse angle.
26
Short Answer
Medium
1 Mark
At the Eco-Friendly Solar Park, an architect designs an A-frame roof where the apex angle measures 72°. Classify this roof angle as acute, right, or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Acute angle
Step-by-Step Explanation:
Since 72° is strictly less than 90°, it is an acute angle.
27
Multiple Choice
Easy
1 Mark
What is the measure of a straight angle?
A
180°
B
90°
C
360°
D
270°
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
180°
Step-by-Step Explanation:
A straight angle forms a flat straight line and measures exactly 180°.
28
Multiple Choice
Easy
1 Mark
An angle measuring 135° is classified as which type of angle?
A
Obtuse angle
B
Acute angle
C
Right angle
D
Straight angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Obtuse angle
Step-by-Step Explanation:
An angle greater than 90° and less than 180° is an obtuse angle.
29
Multiple Choice
Easy
1 Mark
What type of angle is formed by the hands of a clock at 3:00?
A
Right angle (90°)
B
Acute angle
C
Obtuse angle
D
Straight angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Right angle (90°)
Step-by-Step Explanation:
At 3:00, the minute hand points to 12 and the hour hand to 3, forming a 90° right angle.
30
Multiple Choice
Easy
1 Mark
Which type of angle measures strictly less than 90°?
A
Acute angle
B
Obtuse angle
C
Right angle
D
Reflex angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Acute angle
Step-by-Step Explanation:
An acute angle is strictly between 0° and 90°.
31
Short Answer
Hard
2 Marks
At what exact time between 1:00 and 2:00 do the hands of a clock form an acute angle: at 1:10 or at 1:50?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
1:10
Step-by-Step Explanation:
At 1:10 the hands are close together (approx 25° acute angle). At 1:50 the hands are widely spread forming an obtuse/reflex angle.
32
Short Answer
Hard
2 Marks
In a right-angled triangle, one acute angle measures 34°. What is the measure of the other acute angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
56°
Step-by-Step Explanation:
The two acute angles in a right triangle add to 90°. 90° - 34° = 56°.
33
Short Answer
Hard
2 Marks
Explain how you can use the corner of an index card (which is 90°) to determine whether an unknown angle is acute or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Place the index card vertex at the angle's corner with one edge aligned. If the other ray hides inside the card, the angle is acute (90°).
Step-by-Step Explanation:
A standard 90° corner serves as a physical benchmark for angle classification.
34
Short Answer
Hard
2 Marks
Can a triangle have two right angles? Explain using the property of angle sums in triangles.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
No. The sum of all three angles in any triangle is always 180°. Two right angles would total 90° + 90° = 180°, leaving 0° for the third angle, which cannot form a closed triangle.
Step-by-Step Explanation:
Two 90° angles equal 180°, making a third angle impossible in a triangle.
35
Short Answer
Medium
1 Mark
A student claims: 'An obtuse angle can measure 195°.' Explain the error and give the correct classification.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Reflex angle
Step-by-Step Explanation:
Obtuse angles must be less than 180°. An angle greater than 180° is a reflex angle.
36
Short Answer
Medium
1 Mark
Jake looks at a 95° angle and calls it an acute angle because it is close to 90°. Why is Jake mistaken?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
It is an obtuse angle
Step-by-Step Explanation:
Any angle strictly greater than 90° (even 91° or 95°) is classified as obtuse, not acute.
37
Fill in the Blanks
Medium
1 Mark
Find the missing angle to complete a right angle: 38° + [ ? ]° = 90°.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
52
Step-by-Step Explanation:
90 - 38 = 52°.
38
Fill in the Blanks
Medium
1 Mark
Identify the angle classification: Angle X = 90°. Angle X is a [ ? ] angle.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
right
Step-by-Step Explanation:
An angle of exactly 90° is called a right angle.
39
Short Answer
Medium
1 Mark
A carpenter joins two wooden beams forming a straight line of 180°. If one angle is 65°, what is the measure of the adjacent supplementary angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
115°
Step-by-Step Explanation:
Angles on a straight line add to 180°. 180° - 65° = 115°.
40
Short Answer
Medium
1 Mark
Look at an analog clock at 5:00. What type of angle (acute, right, or obtuse) is formed between the hour hand and the minute hand?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Obtuse angle
Step-by-Step Explanation:
5 intervals of 30° = 150°. Since 150° is between 90° and 180°, it is an obtuse angle.
41
Short Answer
Medium
1 Mark
At the Harbor Marine Terminal, an architect designs an A-frame roof where the apex angle measures 72°. Classify this roof angle as acute, right, or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Acute angle
Step-by-Step Explanation:
Since 72° is strictly less than 90°, it is an acute angle.
42
Multiple Choice
Easy
1 Mark
What is the measure of a straight angle?
A
180°
B
90°
C
360°
D
270°
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
180°
Step-by-Step Explanation:
A straight angle forms a flat straight line and measures exactly 180°.
43
Multiple Choice
Easy
1 Mark
An angle measuring 135° is classified as which type of angle?
A
Obtuse angle
B
Acute angle
C
Right angle
D
Straight angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Obtuse angle
Step-by-Step Explanation:
An angle greater than 90° and less than 180° is an obtuse angle.
44
Multiple Choice
Easy
1 Mark
What type of angle is formed by the hands of a clock at 3:00?
A
Right angle (90°)
B
Acute angle
C
Obtuse angle
D
Straight angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Right angle (90°)
Step-by-Step Explanation:
At 3:00, the minute hand points to 12 and the hour hand to 3, forming a 90° right angle.
45
Multiple Choice
Easy
1 Mark
Which type of angle measures strictly less than 90°?
A
Acute angle
B
Obtuse angle
C
Right angle
D
Reflex angle
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Acute angle
Step-by-Step Explanation:
An acute angle is strictly between 0° and 90°.
46
Short Answer
Hard
2 Marks
At what exact time between 1:00 and 2:00 do the hands of a clock form an acute angle: at 1:10 or at 1:50?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
1:10
Step-by-Step Explanation:
At 1:10 the hands are close together (approx 25° acute angle). At 1:50 the hands are widely spread forming an obtuse/reflex angle.
47
Short Answer
Hard
2 Marks
In a right-angled triangle, one acute angle measures 34°. What is the measure of the other acute angle?
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
56°
Step-by-Step Explanation:
The two acute angles in a right triangle add to 90°. 90° - 34° = 56°.
48
Short Answer
Hard
2 Marks
Explain how you can use the corner of an index card (which is 90°) to determine whether an unknown angle is acute or obtuse.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Place the index card vertex at the angle's corner with one edge aligned. If the other ray hides inside the card, the angle is acute (90°).
Step-by-Step Explanation:
A standard 90° corner serves as a physical benchmark for angle classification.
49
Short Answer
Hard
2 Marks
Can a triangle have two right angles? Explain using the property of angle sums in triangles.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
No. The sum of all three angles in any triangle is always 180°. Two right angles would total 90° + 90° = 180°, leaving 0° for the third angle, which cannot form a closed triangle.
Step-by-Step Explanation:
Two 90° angles equal 180°, making a third angle impossible in a triangle.
50
Short Answer
Medium
1 Mark
A student claims: 'An obtuse angle can measure 195°.' Explain the error and give the correct classification.
💡 Click to View Correct Answer & Explanation ▼
Correct Answer:
Reflex angle
Step-by-Step Explanation:
Obtuse angles must be less than 180°. An angle greater than 180° is a reflex angle.