Page 129 - Math Smart - 7

Measuring angles

We use a protractor marked in degrees H to measure angles.

Outer scale
Starts on the left Inner scale
at 0' and goes Starts on the right at 0°
clockwise to 180 and goes anti-clockwise
to 180°.
Baseline shows where 0° and
180 ° are on both scales.
centre

There are two scales on a protractor: an inner scale and an outer scale.
The Inner scale measures in an anti-clockwise direction from 0 on the right-hand
side to 180° on the left-hand side.

The outer scale measures in a clockwise direction from 0 on the left-hand side to
180° on the right-hand side.

Make sure that you read from the correct scale when measuring angles.

Measure this acute angle.

Solution
Place the protractor on the angle. Align the centre of the
protractor with the vertex of the angle as shown. The
baseline of the protractor should line up with one of the
arms of the angle.

Read from the scale that starts at the 0° mark that aligns
with the arm of the angle. Here, we read the inner scale.
The size of this acute angle is 33°.

Example 3

Measure this reflex angle.

Solution
This angle is greater than 180°.
But the protractor can only measure up to 180°.