Primary 4 · Measurement & Geometry
P4 angles
In P4, angles are measured in degrees for the first time. Your child names angles such as ∠ABC, measures them with a protractor and draws an angle of a given size. The protractor's two scales are where the reading slips.
- Taught in
- Term 2
- Syllabus
- 3 skills
- Usual slip
- Reads the other scale
What your child learns in P4 angles
Three skills, each with what it looks like on a worksheet.
See the syllabus wording
- using notation such as ∠ABC and ∠a to name angles
- measuring angles in degrees
- drawing an angle of given size
Naming an angle
∠ABC is the angle at B. The middle letter is always the corner, and the other two letters sit on its arms. An angle can also be named with one small letter, such as ∠x.
Measuring an angle in degrees
Put the protractor's centre on the corner and one arm on 0°. Read the scale that starts at 0 on that arm. Check first: an acute angle is under 90°, an obtuse angle is over 90°.
| Figure | Reading |
|---|---|
| angle under a protractor | measure with the scale |
Drawing an angle of a given size
Draw one arm with a ruler. Put the protractor's centre on one end, with 0 on the arm. Count up from 0 to the size, mark a dot, then join the dot to the corner.
Key facts and formulas
- Naming an angleBy three letters, the corner in the middle: ∠PQR. Or by one: ∠y.
- Measuring with a protractorCentre on the corner, one arm on 0°. Count up from that 0 to the other arm.
Figure Reading angle under a protractor measure with the scale - Drawing an angleDraw one arm. Centre the protractor on its end, 0° on the arm. Mark the size, then join.
Words to know
- Vertex
- The corner where the two arms of an angle meet. In ∠ABC it is B.
- Protractor
- The half-circle tool for measuring angles, with two scales from 0° to 180°.
- Acute angle
- An angle smaller than a right angle, so under 90°.
- Obtuse angle
- An angle bigger than a right angle and smaller than 180°.
The sentence that costs the mark
An angle is measured with a protractor, its centre on the vertex and one arm resting on 0°. What is its size?
| Figure | Reading |
|---|---|
| angle under a protractor | measure with the scale |
What the working usually does
readsthe other scale=120°
Reads the scale that did not start at 0 on the arm. It is a real number at the same mark, so the reading looks done.
What the question asked
readsthe 0° scale=60°
Reads the scale that starts at 0 on the arm along the baseline. Only that scale counts up from the arm.
Both numbers sit at the very same mark, so nothing looks careless. A glance settles it: this angle is smaller than a square corner, so it cannot be 120°.
The same P4 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.
Place the protractor on the marked angle. How many degrees does it measure?
| Figure | Reading |
|---|---|
| angle under a protractor | measure with the scale |
- Line the protractor up. Put its centre on the vertex, with 0° along one arm.
- Follow the other arm out to the scales. It crosses two numbers, one on each scale: 65° and 115°.
- Read the scale that starts at 0° on your first arm. That scale reads 65°. The 115° belongs to the other scale, which starts on the far side.
- The angle is smaller than a right angle, so it must be under 90°. 65° fits, and 115° does not.
- So the marked angle measures 65°.
Every question in StudyLah comes with a walkthrough like this one.
Try it freeThree quick P4 angles questions
Try them together, then tap to check.
Question 1
Use the protractor to measure the angle shown. Write down its size in degrees.
| Figure | Reading |
|---|---|
| angle under a protractor | measure with the scale |
Show answer
Answer58°. Read the scale that starts at 0 on the baseline arm. The angle is under 90°, so 122° cannot be right.
Question 2
In the figure below, which marked angle is greater than a right angle?
Show answer
Answer∠a. It opens wider than a square corner. ∠b, ∠c and ∠d are all narrower, so each is smaller than a right angle.
Question 3
PQRS is a rectangle with a diagonal PR. Find ∠PRQ.
Show answer
Answer55°. ∠PRQ is the angle at R between P and Q. The corner at R is 90°, so ∠PRQ = 90° − 35° = 55°.
Helping at home
How to tell it has clicked
Before your child reads a number, ask whether the angle is acute or obtuse, smaller or bigger than a square corner. If they can name it, a wrong-scale 120° for a small angle gets caught on the spot.
If they read a number straight off without that glance, that is where the mark goes. The fix is a one-second look before the reading.
Try this tonight
Point at corners around the house, a door left open or a book lying open, and ask "more or less than a square corner?" before anyone measures.
If you have a protractor, measure a few of them together. The slip shows up as a small angle reading over 90°, and spotting that mismatch is the skill worth building.
Before and after P4 angles
Builds on
Primary 3 AnglesCompares angles with a square corner
You are here
Primary 4 Angles
Names, measures and draws angles in degrees
Leads to
Primary 5 AnglesFinds unknown angles on lines and points
More in Primary 4
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.