Page 100 - Start Up Mathematics_6

Example 12: Fill in the blanks with acute, obtuse, reflex, right or straight:
(a) The measure of an ______________ angle Do you know?
is less than that of a right angle.

(b) The measure of either an ______________ A dihedral angle is
formed between
or a ______________ angle is greater than two planes.
that of a right angle.
(c) The measure of a ______________ angle
is the sum of the measures of two right angles.

(d) When the sum of the measures of two angles is that of a right angle, then each
one of them is an ______________ angle.

(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 ______________.

Solution: (a) acute (b) obtuse, reflex (c) straight (d) acute (e) obtuse
Example 13: Find the measure of the angle between the hands of the clock in each figure:

(a) (b) (c)

Solution: (a) 90° (b) 60° (c) 120°
60°
Example 14: Draw an angle of measure 60°.
Enlarge it by extending its arms
further. Does the angle become
larger? Does the size of the
angle change?

Solution: On extending the arms further, the
figure looks bigger but the measure
of the angle remains the same.
B

Example 15: Classify each of the angles shown in the given C A
figure as acute, right, obtuse, straight or reflex. O

D

Solution: Angle ∠AOB ∠AOC ∠BOC ∠COD ∠DOA ∠DOB

Classification Obtuse Straight Acute Right Right Reflex

92