
14 Chapter 1
lining up
81º
9º
bº
aº
If we had a pair of right angles, they would add
up to 90º + 90º = 180º.
Our pair of mystery angles make up the same
total angle as a pair of right angles, so
aº + bº = 180º.
Angles on a straight line add up to 180º
Whether a half turn is made up of two quarter turns or
lots of different small turns, the total angle of a half turn
is always 180º.
This means that when the angle on a straight line is
divided up into smaller angles we can always be sure that
they add up to 180º.
Half a pizza is half
a pizza, no matter
how you slice it.
We know that you can always fit a pair of right angles on a
straight line, but what if we don’t have right angles?
What might this pair of mystery angles (a and b) from our
crime scene sketch add up to?
81º
9º
90º + 9º + 81º = 180º
Solution