Primary 5 · Measurement & Geometry
P5 angles
Three rules find most P5 angles: angles on a straight line add up to 180°, angles at a point add up to 360°, and vertically opposite angles are equal. The real skill is reading which one a figure shows.
- Taught in
- Term 3
- Syllabus
- 3 skills
- Usual slip
- Subtracting from 180°
What your child learns in P5 angles
Three skills, each with what it looks like on a worksheet.
See the syllabus wording
- angles on a straight line
- angles at a point
- vertically opposite angles
- finding unknown angles
Angles on a line and at a point
Angles on a straight line add up to 180°. Angles at a point, going all the way round, add up to 360°. The two look alike, so your child checks which one the figure shows before subtracting.
Vertically opposite angles
When two straight lines cross, the angles facing each other are equal: vertically opposite angles. Two adjacent angles, side by side on one line, are supplementary: they add up to 180°.
Find unknown angles, one rule at a time
Your child names each angle with three letters, uses one rule per line and writes the reason beside it, without drawing extra lines. This is called angle chasing, and the mark is for the reason as well as the number.
Key facts and formulas
- Angles on a straight lineThey add up to 180°.a →
- Angles at a pointThey add up to 360°.
- Vertically opposite angles are equalSo a = 110°.
- Angles in a right angleThey add up to 90°.a →
Words to know
- Straight line
- Angles on a straight line add up to 180°.
- At a point
- Angles all the way round a point add up to 360°.
- Vertically opposite
- The angles facing each other where two straight lines cross. They are equal.
- ∠ABC
- The angle at B, between BA and BC. The middle letter is the corner.
- Reason
- The rule a line of working uses, written beside it, such as "angles at a point".
- Reflex angle
- An angle greater than 180°, going the long way round a point.
The sentence that costs the mark
Three angles meet at a point. Two of them add up to 120°. Find the third angle.
What the working usually does
180° −120°=60°
Subtracts from 180°, as if the angles sat on a straight line. They do not, so this lands a full 180° short of the right answer.
What the question asked
360° −120°=240°
Subtracts from 360°, because the rays close all the way round the point. That is the full turn, and the third angle is a reflex angle.
Both are a clean subtraction ending on a real angle, and the figure gives no arithmetic hint. Only reading whether the rays make a straight line or close a point decides which total to use.
The same P5 angles question, worked properly
Lah works through it the way she does every question in StudyLah: one step at a time, marking the picture as she goes.
AOB is a straight line. ∠AOD = 110° and ∠COB = 120°. Find ∠COD.
- AOB is a straight line, so the angles on a straight line add up to 180°, not 360°.
- ∠AOC sits on the straight line beside the angle of 120°, so ∠AOC = 180° − 120° = 60°.
- Now chase the angle we want. The angle of 110° is made of ∠AOC and ∠COD, so ∠COD = 110° − 60° = 50°.
- So the missing angle is 50°.
Every question in StudyLah comes with a walkthrough like this one.
Try it freeTwo quick P5 angles questions
Try them together, then tap to check.
Question 1
PR and ST are straight lines. Which two of the marked angles are equal?
Show answer
Answer∠b and ∠d. They sit across the crossing of PR and ST, so they are vertically opposite angles. ∠a and ∠c each use an extra line.
Question 2
PQ and RS are straight lines. Which is true, ∠e = ∠g or ∠e + ∠f = ∠h?
Show answer
Answer∠e + ∠f = ∠h. Together they are vertically opposite ∠h. ∠e and ∠g only look alike, and no rule makes them equal.
Helping at home
How to tell it has clicked
You do not need to work out the angle to check this. Point at the figure and ask one thing before any subtracting: is there a straight line through the point, or do the angles go all the way round? A child who looks first, then picks 180° or 360°, has it.
Try this tonight
Open a door from shut to flat against the wall. That sweep is 180°, a straight line. Spinning all the way round, back to the start, is 360°. Before your child subtracts anything in an angles question, ask which one the picture is doing.
Before and after P5 angles
Builds on
Primary 4 AnglesNames, measures and draws angles in degrees
You are here
Primary 5 Angles
Finds unknown angles on lines and points
Leads to
Primary 5 Properties of TrianglesUses equal sides to find equal angles
Head Start
How to prepare for PSLE Maths: P5 to P6 revisionOne of five P5 topics to revise before P6 Term 1
Leads to
Primary 6 Angles in Geometrical FiguresFinds unknown angles in composite figures
More in Primary 5
Struggling with the working rather than the topic itself? How to help your child with problem sums
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.