Chapter 5 Understanding Elementary Shapes Ex. 5.4
What is the measure of (i) a right angle (ii) a straight angle?
(i) Measure of a right angle = 90°
(ii) Measure of a straight angle = 180°
Say True or False:
(a) The measure of an acute angle < 90°
(b) The measure of an obtuse angle < 90°
(c) The measure of a reflex angle > 180°
(d) The measure of one complete revolution = 360°
(e) If m ∠A = 53° and ∠B = 35°, then m∠A > m∠B.
Write down the measures of
(a) some acute angles
(b) some obtuse angles
(a) 25°, 63° and 72° are acute angles.
(b) 105°, 120° and 135° are obtuse angles.
Measure the angles given below using the protractor and write down the measure.
(d) ∠1 = 60°, ∠2 = 90°, ∠3 = 125°
Which angle has a large measure? First estimate and then measure.
Measure of Angle A =
Measure of Angle B =
Measure of Angle A = 40°
Measure of Angle B = 60°.
From these two angles which has large measure? Estimate and then confirm by measuring them.
The opening of angle (b) is more than angle (a).
∴ Measure of angle (a) = 45°
and the measure of angle (b) = 60°
Fill in the blanks with acute, obtuse, right or straight:
(a) An angle whose measure is less than that of a right angle is ……… .
(b) An angle whose measure is greater than that of a right angle is ……… .
(c) An angle whose measure is the sum of the measures of two right angles is ……… .
(d) When the sum of the measures of two angles is that of a right angle, then each one of them is ……… .
(e) When the sum of the measures of two angles is that of a straight angle and if one of them is acute then the other should be ……… .
Find the measure of the angle shown in each figure. (First estimate with your eyes and than find the actual measure with a protractor).
(a) Measure of the angle = 40°
(b) Measure of the angle = 130°
(c) Measure of the angle = 65°
(d) Measure of the angle = 135°.
Find the angle measure between the hands of the clock in each figure:
(i) The angle between hour hand and minute hand of a clock at 9.00 a.m = 90°
(ii) The angle between the hour hand and minute hand of a clock at 1.00 p.m = 30°
(iii) The angle between the hour hand and minute hand of a clock at 6.00 p.m = 180°.
Investigate: In the given figure, the angle measures 30°. Look at the same figure through a magnifying glass. Does the angle becomes larger? Does the size of the angle change?
It is an activity. So try it yourself.
Measure and classify each angle: