How_To_Be_Good_At_Math

GEOMETRY • USING A PROTRACTOR 239

Measuring angles up to 180°

You can use a protractor to measure

the angle formed by any two lines.

Put the centre of

the protractor

over the vertex

Use the inner

scale to measure

the smaller angle

Use the outer

scale to measure

the larger angle

Use a ruler and pencil to

extend the angle's arms if

you need to. This makes it easier

to read the angle.

Put the protractor along one

arm of the angle. Take a

reading from where the other

arm crosses the protractor.

To measure the larger angle,

read up from zero on the

other side of the protractor.

When we measure angle b,

we find that it's 60°.

We know there are 360° in

a full turn. So, angle a must

be 360° − 60°.

To find angle a, put the

protractor along one arm, facing

downwards.

So, the answer is a = 300°.

Make the arms longer

if they’re not long

enough to read

Measure the angles

Practise your protractor skills by

measuring these angles. It helps

to estimate angles before

measuring them – that way,

you'll make sure you read from

the correct scale.

TRY IT OUT

Place the

protractor's

centre over

this point

Make sure you

read from the

correct scale

Measuring reflex angles

Reflex angles are angles larger than 180°. We can use a

semicircular protractor to measure a reflex angle if we combine

our measurements with what we know about calculating angles.

b = 60°

a

b

0° 0°

Answers on page 320

238_239_Using_a_protractor.indd 239 29/02/2016 18:05