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Primary 4 Mathematics · Chapter 7 · Answers and explanations

Lesson 7.3: Measuring Angles (answers)

Section A · Multiple choice 4 marks

1. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 40°
(A) 140°(B) 50°(C) 130°(D) 40°
Answer: (D) 40°One arm is on 0 at the right, so read the outer scale that starts at 0 on the right. The other arm is at 40°.

2. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 65°
(A) 65°(B) 115°(C) 55°(D) 125°
Answer: (A) 65°One arm is on 0 at the LEFT, so read the inner scale that starts at 0 on the left. The other arm is at 65°. (115° is the reading on the other scale.)

3. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 135°
(A) 45°(B) 135°(C) 125°(D) 145°
Answer: (B) 135°The arm is on 0 at the right, so use the outer scale: 135°. The angle is wider than a square corner, so it is obtuse and 45° cannot be right.

4. When Sam measured an acute angle, he read 30° on one scale and 150° on the other scale. What is the size of the angle?1 mark

(A) 150°(B) 60°(C) 30°(D) 120°
Answer: (C) 30°An acute angle is less than 90°, so the size must be 30°.

Section B · Write your answer 11 marks

5. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 70°
Answer: 70°Arm on 0 at the right, so read the outer scale. The other arm is at 70°.

6. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 35°
Answer: 35°Arm on 0 at the left, so read the inner scale. The other arm is at 35°.

7. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 115°
Answer: 115°Arm on 0 at the right, so read the outer scale: 115°. It is an obtuse angle.

8. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 120°
Answer: 120°Arm on 0 at the left, so read the inner scale: 120°. It is an obtuse angle.

9. What is the size of the angle?1 mark

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800ABC∠ABC = 85°
Answer: 85°Arm on 0 at the left, so read the inner scale: 85°. It is an acute angle.

10. Measure ∠XYZ. Is it acute, right or obtuse?2 marks

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800XYZ∠XYZ = 150°
Answer: 150°; obtuseStart at 0 on the outer scale (the arm YX is on the right side). The other arm is at 150°. It is more than 90°, so it is obtuse.

11. Lina says ∠PQR is 110° because the arm is at 110 on the outer scale. Is she correct? What is the size of ∠PQR?2 marks

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800PQR∠PQR = 70°
Answer: No; ∠PQR = 70°One arm is on 0 at the left, so Lina must use the inner scale, which gives 70°. The angle is acute (smaller than a square corner), so it cannot be 110°.

12. The two arms OB and OC are drawn from O. Find the size of ∠BOC.2 marks

01801017020160301504014050130601207011080100909010080110701206013050140401503016020170101800AOBC∠BOC = 105° − 35° = 70°
Answer: 70°OB is at 35° and OC is at 105° on the outer scale. ∠BOC = 105° − 35° = 70°.
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