Primary 4 Mathematics · Chapter 7 · Answers and explanations
Lesson 7.1: Recall: Acute, Right and Obtuse Angles (answers)
Section A · Multiple choice 4 marks
1. What kind of angle is shown?1 mark
(A) a right angle(B) an acute angle(C) an obtuse angle(D) a straight line
Answer: (B) an acute angleIt is smaller than a right angle (a square corner), so it is an acute angle.
2. What kind of angle is shown?1 mark
(A) an acute angle(B) a right angle(C) an obtuse angle(D) a straight line
Answer: (C) an obtuse angleIt is bigger than a right angle but smaller than a straight line, so it is an obtuse angle.
3. A right angle is the same as a turn of1 mark
(A) a half turn(B) three-quarters of a turn(C) a full turn(D) a quarter turn
Answer: (D) a quarter turnA full turn is 360°. A quarter turn is 360° ÷ 4 = 90°, which is a right angle.
4. Which angle is the biggest?1 mark
(A) Angle Y(B) Angle X(C) Angle Z(D) They are all the same
Answer: (A) Angle YAngle Y opens the widest, so it is the biggest. (X is 65°, Y is 140° and Z is 100°.)
Section B · Write your answer 11 marks
5. Is an angle of 150° acute, right or obtuse?1 mark
Answer: obtuseAn acute angle is less than 90° and a right angle is exactly 90°. 150° is more than 90° and less than 180°, so it is obtuse.
6. How many right angles make a half turn?1 mark
Answer: 2A half turn is 180°. 90° + 90° = 180°, so 2 right angles make a half turn.
7. How many right angles does a rectangle have?1 mark
Answer: 4All 4 corners of a rectangle are square corners (right angles).
8. The minute hand of a clock moves from 12 to 3. How many degrees does it turn?1 mark
Answer: 90°From 12 to 3 is a quarter turn, which is 90°.
9. Write the numbers of the obtuse angles.1 mark
Answer: Angles 3 and 4Angle 1 is smaller than a square corner (acute). Angle 2 is a square corner (right). Angles 3 and 4 are wider than a square corner but not a straight line, so they are obtuse.
10. Write acute, right or obtuse for each angle: X, Y and Z.2 marks
Answer: X: obtuse, Y: acute, Z: rightX is wider than a square corner (obtuse). Y is narrower than a square corner (acute). Z is exactly a square corner (right).
11. The minute hand of a clock moves from 12 to 3 and then from 3 to 6. How many degrees has it turned altogether? What turn is this?2 marks
Answer: 180°; a half turn12 to 3 is 90° and 3 to 6 is 90°. 90° + 90° = 180°, which is a half turn.
12. Write these angles in order, from the smallest to the biggest: 130°, 45°, 90°, 170°. Which of them are obtuse angles?2 marks
Answer: 45°, 90°, 130°, 170°; obtuse: 130° and 170°Order them: 45°, 90°, 130°, 170°. Obtuse angles are more than 90° and less than 180°, so 130° and 170° are obtuse. 45° is acute and 90° is a right angle.
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