Page 110 - classs 6 a_Neat

(iii) OBTUSE ANGLE An angle whose measure is more than go but less than 180° is called an obtuse angle. EXAMPLE 1: Find the measure of each one of the angles given below by using a protractor.
o
In the adjoining figure, POQ is an obtuse angle.

(iv) STRAIGHT ANGLE An angle whose measure is 180° is called a straight angle.
In the gtven figure, XOY = 180° = a straight angle.

(v) REFLEX ANGLE An angle whose measure is more than 180° but less than 360° is called a reflex angle.
In the adjoining figure, AOB is a reflex angle.

(vi) COMPLETE ANGLE An angle whose measure ts 360° ts called a complete angle.
In the adjoining figure, AOA = 360° .
Method (i) In order to measure AOB, place the protractor in such a way that its centre is
exactly on the vertex O of the angle and the base line lies along the arm OA. Read
off the mark through which the arm OB passes, starting from 0° on the side of A, as
O
(Complete angle) A shown in the figure.

Thus, we find that AOB = 38°.
(VII) ZERO ANGLE The angle whose masure ts 0° ts caled a zero angle.
In the given figure, AOA = 0°.

O A

MEASURING AN ANGLE BY A PROTRACTOR

PROTRACTOR It is an instrument for measuring or constructing an angle of a given measure. It is a semi-
circular piece of metal or plastic, which is marked in degrees from 0° to 180° from left to right as well as from
right to left, as shown in the figure. 0 A

(ii) In order to measure PQR, place the protractor in such a way that its centre is
exactly on the vertex Q of the angle and the base line lies along the arm PQ. Read off the
mark through which the arm QR passes, starting from 0° on the side of P, as shown in
the figure.
Thus, we find that PQR = 127°.

The centre O of this piece is also the midpoint of its base line.