Page 55 - Cambridge Checkpoint Mathematics Coursebook 7_Slide 02
P. 55

5 Angles







               When you measure lengths you use diff erent units, including          Key words
                                                                                    Key words
                                                                                    Key words
               millimetres, metres and kilometres. But length is not the only type
               of measurement you need to make when you are looking at fl at       Make sure you learn and
               shapes. Sometimes, you need to change direction, for example, if    understand these key words:
               you turn a corner.                                                  angle
               A turn between one direction and another is called an angle. You    degree
               measure angles in degrees. One whole turn is 360 degrees, written   right angle
               as 360°.                                                            acute angle
               Humans have needed to measure angles for a long time. When          obtuse angle
                                                                                   line segment
               early astronomers looked at stars in the sky, they wanted to        refl ex angle
               describe their positions relative to one another. Th  e natural way to   protractor
               do this was to use angles. We know that the ancient Babylonians     equilateral
               and Egyptians divided a whole turn into 360 parts, as long ago as   quadrilateral
               1500 BC.                                                            vertically opposite angles

                                                                                   perpendicular
               Why are there 360 degrees in a whole turn?                          parallel
               A clay tablet excavated in Shush, in what is now modern-day Iran,   transversal
               shows that Babylonians divided a whole turn into 360 units. One     isosceles
               reason could be that many simple fractions of a whole turn of 360°,

                         1 1 1 1
                                        1
               including  ,,    ,  and  , can be written as a whole number of
                         2 3 4 5        6
               degrees. It may also be because there are approximately
               360 days in a year.
               The angles of a triangle
               Th  e sum or total of the angles of a triangle is always 180°.








               In this unit you will learn about other angle facts and use them to
               solve problems.

                                                                                     Ancient Babylonian tablet
                                                                                     recording measurements.
















                                                                                                           5 Angles     53
   50   51   52   53   54   55   56   57   58   59   60