Page 111 - Start Up Mathematics_6

At a Glance

1. Magnitude of the angle is the amount of rotation through which one of the arms is moved
about the vertex to bring it to the position of the other arm.
2. Some special angles are:

(i) Zero angle = 0° (ii) Right angle = 90°
(iii) Straight angle = 180° (iv) Complete angle = 360°

3. Different kinds of angles:
(i) Acute angle — measures more than 0° but less than 90°.
(ii) Obtuse angle — measures more than 90° but less than 180°.
(iii) Reflex angle — measures more than 180° but less than 360°.

4. Two lines are perpendicular if they intersect at right angles.
5. Triangles can be classified in two ways:
(i) According to sides
(a) Scalene — A triangle in which all the sides are of different length
(b) Isosceles — A triangle in which two sides are of equal length
(c) Equilateral — A triangle having all sides of equal length
(ii) According to angles
(a) Acute-angled — A triangle in which all the angles are acute
(b) Right-angled — A triangle in which one angle is 90°
(c) Obtuse-angled — A triangle in which one angle is obtuse
6. Quadrilaterals—A four sided polygon is called a quadrilateral. These can be classified
according to the properties they possess.
(i) Parallelogram — A quadrilateral in which opposite sides are parallel
(ii) Rectangle — A parallelogram with a right angle
(iii) Square — A rectangle with a pair of adjacent sides equal
(iv) Rhombus — A parallelogram with all sides equal
(v) Trapezium — A quadrilateral having one pair of parallel sides
(vi) Kite — A quadrilateral having two distinct pairs of equal adjacent sides
7. A polygon is said to be regular if all its sides and angles are equal.

Review Exercises

Multiple ChoiCe Questions
1. The adjacent sides of a rectangle are:
(a) parallel (b) perpendicular (c) coincident (d) concurrent
2. The angle by which the hour hand of a clock turns through when the minute hand goes from
12 p.m. to 6 p.m. is:

(a) 15° (b) 60° (c) 90° (d) 180°
3. Number of right angles turned if you are facing south and turn anticlockwise to face west is:
(a) 1 right angle (b) 2 right angles (c) 3 right angles (d) 4 right angles
4. An angle whose measure is greater than 90° cannot be:
(a) obtuse (b) reflex (c) straight (d) acute

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