1. What is the measure of
(i) a right angle? (ii) a straight angle?
Ans.
(i) The measure of a right angle is 900
(ii) The measure of a straight angle is 1800
2. Say True or False :
(a) The measure of an acute angle < 90°.
Ans. True
Reason – The measure of an acute angle < 90o.
(b) The measure of an obtuse angle < 90°.
Ans. False
Reason – The measure of an obtuse angle > 90o but less than 180o.
(c) The measure of a reflex angle > 180°.
Ans. True
Reason – The measure of a reflex angle is more than 1800.
(d) The measure of one complete revolution = 360°.
Ans. True
Reason – The measure of one complete revolution = 360°.
(e) If m ∠ A= 53° and m∠ B = 35°, then m A∠ > m ∠B.
Ans. True.
Reason – ∠A is greater than ∠B.
3. Write down the measures of (give at least two examples of each).
(a) some acute angles.
Ans. Any angle having measure less than 90o is an acute angle. Therefore, measure of some acute angles is – 89o, 55o etc.
(b) some obtuse angles.
Ans. Any angle having measure more than 90o but less than 180o is an obtuse angle. Therefore, measure of some acute angles is – 91o, 119o, 155o etc.
4. Measure the angles given below using the Protractor and write down the measure.
5. Which angle has a large measure?
First estimate and then measure.
Measure of Angle A =
Measure of Angle B =
Sol. The measure of angle A shown above is 400
While the measure of angle B shown above is 680 Therefore, ∠B > ∠A
6. From these two angles which has larger measure? Estimate and then confirm by measuring them.
Sol. The measure of angle shown above in the first figure is 450
While the measure of angle shown above in the first figure is 550 Therefore, angle shown in the second figure has large measure.
(7) Fill in the blanks with acute, obtuse, right or straight :
(a) An angle whose measure is less than that of a right angle is______.
Ans. Acute Angle
Reason – The measure of an acute angle < 90o.
(b) An angle whose measure is greater than that of a right angle is ______.
Ans. Obtuse Angle
Reason – The measure of an obtuse angle > 90o but less than 180o.
(c) An angle whose measure is the sum of the measures of two right angles is _____.
Ans. Straight angle
Reason – sum of two right angle means 90o + 90o = 180o (straight angle).
(d) When the sum of the measures of two angles is that of a right angle, then each one of them is ______.
Ans. Acute angle
Reason – Sum of two angle is 90o. It is possible only when each one of them is less than 90o (Acute angles)
(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 _______.
Ans. Obtuse angle
8. Find the measure of the angle shown in each figure. (First estimate with your eyes and then find the actual measure with a protractor)
Ans. The measures of the angle shown above in first figure is 400
The measures of the angle shown above in second figure is 1300
The measures of the angle shown above in third figure is 650
The measures of the angle shown above in forth figure is 1350
9. Find the angle measure between the hands of the clock in each figure:
Ans.
The measures of the angle shown above in first clock showing 9’o clock is 900
The measures of the angle shown above in second clock showing 9’o clock is 300
The measures of the angle shown above in third clock showing 9’o clock is 1800
10. Investigate
In the given figure, the angle measures 30°. Look at the same figure through a magnifying glass. Does the angle become larger? Does the size of the angle change?
Ans. No, because even if we look any angle in the magnifying glass, the measure of the angle will not change. Only the angle will look bigger in size but its measure will not differ.
11. Measure and classify each angle
