The idea in brief. A name says WHICH angle, and a bracket says WHICH property. The middle letter of a three-letter name is the vertex and the outer two are the lines the angle sits between, so `∠MON` picks one angle out of the ten at a crossing; and every deduction is one written line with the property that produced it in brackets underneath. Those two conventions are the whole of what the rest of the chapter writes. In the lesson. The bracket says which property.
Lesson video, with the working
Angles on a straight line
2 min 41 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
∠ROJ = 180° − 42° − 67° − 31° = 40° (angles on a straight line KOJ)
Answer: 40°
For parents
How do you answer basic angle properties questions in P6?
This opening page gathers the angle facts a Primary 6 child needs before any composite figure: how an angle is named, and the properties of lines and points.
What your child learns here
This opening page gathers the angle facts a Primary 6 child needs before any composite figure: how an angle is named, and the properties of lines and points. Each line of working gets its reason written in words in a bracket.
Name an angle with three letters, such as ∠ABC, where the middle letter is the vertex.
Use angles on a straight line, which add up to 180°.
Use angles at a point, which add up to 360°, and vertically opposite angles, which are equal.
Read a small square at a corner as a right angle of 90°, and test whether two lines are perpendicular by looking for a right angle.
Does the answer make sense?
An angle your child finds on a straight line must be smaller than 180°, and one found round a point must be smaller than 360°. Where two straight lines cross, the angles facing each other must come out equal, so a facing pair with different sizes has been mixed up with neighbouring angles.
How to practise these questions
Basic questions train angles on a straight line, angles at a point, vertically opposite angles, and judging whether two lines are perpendicular. Challenge questions mark equal angles with letters, give a reflex angle or describe one angle as twice another, so your child sets up the unknowns before calculating. When naming or finding an angle on lines feels stuck, read the written step-by-step solution.
Questions parents ask
Why does my child have to write a reason in brackets? The reason, such as (angles on a straight line), shows which property each line uses. Our lessons write it in full words beside every line that uses a property, which also makes the working easy to check later.
What is the middle letter in an angle name for? In ∠ABC the middle letter B is the vertex, the point where the two arms meet, and the other two letters name the arms. The syllabus introduces this notation in earlier years, and P6 angle questions use it throughout.
How can my child tell if two lines are perpendicular? Two lines are perpendicular when they meet at a right angle, 90°. If one angle where they cross is 90°, all four are. Looking square on the drawing is not enough, because figures are not always drawn to scale.
