How_To_Be_Good_At_Math

236 GEOMETRY •^ OPPOSITE ANGLES

Opposite angles

When two straight lines cross, or intersect, they create two

pairs of matching angles called opposite angles. We can

use this information to work out angles we don't know.

70º 70º

c

70º

a

110º b d

Use a protractor to

measure the angle

The second pair of

opposite, equal

angles is coloured

green

The opposite angles

are coloured the

same blue to show

they are equal

If we call the angles a, b, c, and d,

we can write what we know about

opposite angles like this:

a = c b = d

Now let's look at the other pair

of opposite angles. When we

measure them, we find that they are

also equal – they are both 110°.

When we measure the top angle, we

find it's the same as the bottom one.

The angles opposite each other are equal.

Let's look at what's special about

opposite angles. First, we draw two

intersecting straight lines, then measure

the bottom angle.

When two lines

intersect, the angles

directly opposite each

other are always equal.

110º

236_237_Opposite_angles.indd 236 29/02/2016 18:05