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Primary 4 Mathematics · Chapter 7 · Answers and explanations

Lesson 7.2: Naming Angles (answers)

Section A · Multiple choice 4 marks

1. Which is the correct name of the marked angle?1 mark

ACB∠ABC (vertex B in the middle)
(A) ∠BAC(B) ∠CAB(C) ∠ABC(D) ∠BCA
Answer: (C) ∠ABCThe vertex is B, where the two arms meet. The vertex letter goes in the middle, so the angle is ∠ABC.

2. In ∠PQR, which letter is the vertex?1 mark

(A) P(B) R(C) P and R(D) Q
Answer: (D) QThe vertex is the middle letter, so it is Q.

3. Which angle is marked in the triangle?1 mark

XYZ∠XZY: vertex Z is in the middle
(A) ∠XZY(B) ∠ZXY(C) ∠XYZ(D) ∠YXZ
Answer: (A) ∠XZYThe marked angle is at Z, so Z is the middle letter. It is ∠XZY (or ∠YZX).

4. Which angle is the same as ∠DEF?1 mark

(A) ∠EDF(B) ∠FED(C) ∠DFE(D) ∠EFD
Answer: (B) ∠FED∠DEF and ∠FED both have E in the middle, so they are the same angle.

Section B · Write your answer 11 marks

5. Name the marked angle.1 mark

MKN∠MNK or ∠KNM
Answer: ∠MNK (or ∠KNM)The arms meet at N, so N goes in the middle: ∠MNK or ∠KNM.

6. The vertex of ∠STU is a point. Which letter names it?1 mark

Answer: TThe vertex is the middle letter, T.

7. Write another name for ∠ABC.1 mark

Answer: ∠CBASwap the first and last letters and keep the vertex B in the middle: ∠CBA.

8. ABCD is a rectangle. Name the marked angle.1 mark

ADCB∠ADC or ∠CDA
Answer: ∠ADC (or ∠CDA)The marked angle is at corner D, so D goes in the middle: ∠ADC or ∠CDA.

9. True or false: ∠ABC and ∠ACB are the same angle.1 mark

Answer: False∠ABC has its vertex at B but ∠ACB has its vertex at C, so they are different angles.

10. Name the three angles of triangle PQR that are marked.2 marks

PQR∠QPR, ∠PQR, ∠PRQ
Answer: ∠QPR, ∠PQR, ∠PRQ (other orders of the outer letters are also correct)Write the vertex in the middle each time. At P: ∠QPR. At Q: ∠PQR. At R: ∠PRQ.

11. Name all the angles in the figure. Which one is the obtuse angle?2 marks

ABCOAOB 50°, BOC 80°, AOC 130°
Answer: ∠AOB, ∠BOC, ∠AOC; ∠AOC is obtuseAll the angles have O as the vertex: ∠AOB, ∠BOC and ∠AOC. ∠AOC is the widest one and is more than a right angle, so it is obtuse.

12. Mei says the marked angle is ∠ABC. Raj says it is ∠BAC. Who is correct? Give a reason.2 marks

ABC∠ABC (B is the vertex)
Answer: Mei is correctThe marked angle is at B, and the vertex letter must be in the middle. So it is ∠ABC. ∠BAC has its vertex at A.
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