Year 7
Read and draw angles with a protractor accurately, recognise angle types from acute to full turn, and use angle facts to solve missing-angle problems.
Geometry - angle measure and construction
Angles measure turn. We measure them in degrees using a protractor, placing the centre on the vertex and reading from the correct zero scale. Before measuring, it helps to estimate the angle type. Acute, right, obtuse, straight, reflex and full-turn angles each show a different amount of turn. Once an angle is measured or drawn accurately, angle facts such as angles on a straight line summing to 180 deg and angles at a point summing to 360 deg help us reason about unknown angles.
Start from the correct zero
A protractor has two number scales. Choose the scale that starts where the arm of the angle starts.
Place the centre on the vertex
Line up one arm with zero
Read the matching scale
Most measuring errors happen because the wrong scale is read.
Angle types from small turns to full turns
Estimate the type first so the measured value feels reasonable.
Estimate first, then measure or draw carefully with the protractor.
Use angle totals to find missing values
If one angle on a straight line is 65 deg, the other must be 115 deg.
Subtract the known total from 180 deg or 360 deg depending on the diagram.
An angle on a straight line measures 72 deg. Find the missing angle.
| Known angle | Whole line | Missing angle |
|---|---|---|
| 72 deg | 180 deg | 108 deg |
Estimate the angle type first, then use accurate measuring or angle facts.
What is the missing angle on a straight line if the other angle is 48 deg?
Which card shows a reflex angle?
If three angles at a point are 110 deg, 95 deg and x, what is x?
Which statement is correct when measuring an angle with a protractor?
on the vertex
on zero
use matching scale
What angle should you draw if asked for an obtuse angle?
Watch for these when working through the lesson.
These ideas fit closely with this lesson.