Angles
Definition
An angle is the amount of turning between 2 directions. That is, clockwise or anti clock wise. Angles are measured in degrees (o).
One complete circle = 180o
Half a circle = 180o
An acute circle < 90o
An obtuse angle > 90o
A reflex angle > 180o
Protractor:
It is an instrument used to measure right angles. The correct use of a protractor is as shown below;
- Look at the angle, roughly estimate its size. Decide whether it is < or > 90o OR < or > 180o
- Place the protractor on the angle with its center on the point of the angle (vertex). Its zero line should be along a line of the angle.
- Find the zero degree (0o) on top of the arm of the angle. Ground its scale until you reach the other arm, Count the number on this scale as you go. 0o, 10o, 20o, … Then any extra degrees, 1o, 2o, 3o…
- Compare your measurement with the estimate to draw the angle
- Use your ruler to draw a straight line. Mark a point on it where you want the angle point (vertex) to be. Leave enough space for the angle.
- Put the center of the protractor on the point. Line up its zero line on top of the line you draw
- Keep the protractor still. Count round its scale from 0o on the line you draw, to the size you want. Put a dot next to it.
- Remove the protractor, draw a straight line through the point and the dot.
Ex: 1) name the following angles a)95o, b) 181o, c)53o, d)2o
2) Find the value of x in the figure below
20o
x
Solution: 1) a) obtuse angle, b) reflex angle, c) acute angle, d) acute angle
2) x + 20 = 90, x = 90 – 20, x = 70o
