Page 11 - 4-Math-8 Geometry
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8.3 Measuring and drawing angles
We measure angles in degrees ( ).
There are 90 in a right angle.
There are two right angles in a straight line.
A straight line is 180. 90 90
90 +90 180 90 90
90 90
A complete circle is the same as four right angles or 360.
90 + 90 +90 +90 360
We can say that 1 th part of complete circle is equal to one degree.
360
An angle less than 90 is an acute angle.
An angle greater than 90 but smaller than 180 is an obtuse angle.
Look at the angle ABC. A
BC
Let’s measure ABC. upper scale lower scale70 80 90 100
110 100 90 80 110 61020 50130
We can use a protractor to 50130 61020 70
measure the angle.
15300 14040 14040
15300
20160 160
20
10 170
170 10
0 180
180 0
centre mark
A protractor has a centre mark, an upper scale and a lower scale.
Upper scale reads the measure of angles from left to right.
Lower scale reads the measure of angles from right to left.
Each small marking on the scales shows 1 degree ( 1 ).
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