Angles on a straight line, and vertically opposite angles is in the Primary 5 Angles chapter. The lesson film is the one Mathweave teaches this skill with in Primary 6, and the practice set below is written for Primary 5.
Lesson video, with the working
Straight lines and opposite angles
2 min 46 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
Question 1 of 6 · Intermediate · free to tryXOY is a straight line. Ray OU lies between OX and OV, and OV lies between…XOY is a straight line. Ray OU…
XOY is a straight line. Ray OU lies between OX and OV, and OV lies between OU and OY. Angle XOU is a right angle and angle UOV = 34°. (a) Find angle XOV. (b) Find angle VOY.
(a) ∠XOV = 90° + 34° = 124° (the two parts of ∠XOV)
(b) ∠VOY = 180° − 124° = 56° (angles on a straight line XOY)
Answer: (a) 124°; (b) 56°
For parents
How do you solve P5 angles on a straight line questions?
This page covers two angle facts from the Primary 5 Geometry strand: angles on a straight line add up to 180°, and vertically opposite angles are equal.
What your child learns here
This page covers two angle facts from the Primary 5 Geometry strand: angles on a straight line add up to 180°, and vertically opposite angles are equal. Your child uses them to find unknown angles and names the property behind each step.
Angles on a straight line add up to 180°, however many angles share the line.
Where two straight lines cross, the angles facing each other across the crossing point are vertically opposite angles, and they are equal.
An unknown angle on a line is 180° minus all the known angles on that line.
When the angles on a line are described in equal parts, one part is found from 180° first, then the angle asked for is built from the parts.
Each line of working carries its reason in brackets, for example (angles on a straight line) or (vertically opposite angles).
Does the answer make sense?
Any unknown angle your child finds on a straight line must be less than 180°, and together with its neighbours on that line it fills exactly 180°. When the angles are given as a ratio, the biggest share of the ratio must also be the widest angle on the line.
How to practise these questions
The Challenge questions add angles on a straight line and check the sum against a right angle, or share the straight angle among several angles in a given ratio. If the angle facts get tangled, watch that question's solution film and try it again from Revision later. Multiple-choice questions ask which statement is true about the angles formed where two straight lines cross, at Basic and Intermediate, or about angles on a straight line, at Basic; these are solved in writing rather than by film.
Questions parents ask
Is angles on a straight line in the Primary 5 syllabus? Yes. The Primary 5 Geometry strand lists angles on a straight line, angles at a point and vertically opposite angles, and finding unknown angles with them.
Does my child need to write the reason for each angle? Writing the property beside each step, such as (angles on a straight line), shows which fact was used. The worked example on this page writes the reason in brackets on every line, and it is a good habit to copy.
Can my child measure the angle with a protractor instead? Not unless the question asks for measuring. A figure may not be drawn to scale, so the angle is worked out from the angles given.
