Measurement Of Angle
An angle is the figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle.
Unit of measurement of angle is degrees.
A circle is divided into 360 equal degrees, so that a right angle is 90°. For the time being, we will only consider angles between 0° and 360°, but later, in the section on trigonometric functions, we will consider angles greater than 360° and negative angles.
Right angle is an angle of 90°, as in a corner of a square, or formed by dividing a circle into quarters.
An angle smaller than a right angle is called an acute angle.
If an angle is larger than a right angle, but less than a straight angle, it is called an obtuse angle.
Consider the following diagram and let us see how to measure its angle
1. Place the protractor so that the mid point of its straight edge lies on the vertex B of the angle.
2. Adjust the protractor so that BC is along the straight-edge of the protractor.
3. There are two ‘scales’ on the protractor: read that scale which has the 0° mark coinciding with the straight-edge .
4. The mark shown by BA on the curved edge gives the degree measure of the angle. We write m ∠ABC= 40°, or simply ∠ABC= 40°.