lining up Solution 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? 9º 81º 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. bº 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. 90º + 9º + 81º = 180º 9º 81º 14 Chapter 1