234 GEOMETRY • ANGLES ON A STRAIGHT LINE
Imagine that your line made a stop
on the way to the half turn, creating
an extra line. The two angles made by
the new line add up to 180°
If we rotated a line halfway round from
where it started, the line would turn
180° and it would make a straight line.
No matter how many angles you
create on a straight line, they will
add up to 180°, as long as all the lines
start from the same point.
If the angles on a straight line are
called a and b, we can write this rule
as a formula:
a + b = 180°
Angles on a
straight line
Sometimes, simple rules can help us work out unknown angles.
One of these rules is about the angles that make up a straight line.
Finding a missing angle on a straight line
We know that the three
angles on the straight line
add up to 180°.
Let’s use the rule we’ve just
learned to find the missing
angle on this straight line.
We know that one angle is
45° and the other is 55°.
Let’s add the angles together:
45° + 55° = 100°
Now let’s subtract that total
from 180°:
180° − 100° = 80°
So the missing angle is 80°.
180º
70º
50º
60º
120º 60º
a b
Angles on a straight
line always add up
to 180°.
45º
55º
?
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