CLASS: VI NOTES SUB: MATHEMATICS

CHAPTER 5: UNDERSTANDING ELEMENTARY SHAPES (TERM –II, PART -4)

THIS CHAPTER IS FOR SUBJECT ENRICHMENT ACTIVITY.

ANSWERS/SOLUTIONS CAN BE NOTED IN THE TEXT BOOK ITSELF.

ANGLES (CONTD.) Exercise 5.4

1. What is the measure of:

(i) A right angle? Ans: (i) The measure of a right angle is 90⁰.

(ii) A straight angle? Ans: The measure of a straight angle is 180⁰.

2. Say True or False:

(a) The measure of an acute angle < 90⁰. (True)

(b) The measure of an obtuse angle < 90⁰. (False)

(c) The measure of a reflex angle > 180⁰. (True)

(d) The measure of one complete revolution = 360⁰. (True)

(e) If m ∠A = 53⁰ and m ∠B = 35⁰, then m ∠A > m ∠B. (True)

3. Write down the measures of:

(a) Some acute angles. Ans: 50⁰, 65⁰.

(b) Some obtuse angles. Ans: 110⁰, 175⁰.

4. Measures the angles given below using the protractor and write down the measure.

Sol: (a) The measure of an angle is 45⁰. (c) The measure of an angle is 90⁰.

(b) The measure of an angle is 120⁰. (d) The measures of an angles are 60⁰,90⁰ &130⁰.

5. Which angle has a large measure? First estimate and then measure

A = 40⁰ B = 65⁰ ANGLE B HAS LARGER MEASURE

6. From these two angles which has larger measure? Estimate and then confirm by measuring

them.

Sol: The measures of these angles are 450 & 550. Angle shown in second figure is greater.

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 acute angle.

(b) An angle whose measure is greater than that of a right angle is obtuse angle.

(c) An angle whose measure is the sum of the measures of two right angles is straight angle.

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

is acute 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 obtuse angle.

8. Find the measure of the angle shown in each figure.

Sol: The measures of the angles shown in above figure are 40⁰, 130⁰, 65⁰ and 135⁰.

9. Find the angle measure between the hands of the clock in each figure:

Sol: The angle measure between the hands of the clock are 90⁰, 30⁰ and 180⁰.

11. Measure and classify each angle:

Angle Measure Type

∠AOB 40⁰ Acute

∠AOC 125⁰ Obtuse

∠BOC 85⁰ Acute

∠DOC 95⁰ Obtuse

∠DOA 140⁰ Obtuse

∠DOB 180⁰ Straight

PERPENDICULAR LINES

Perpendicular lines: When 2 lines intersect and the angle between them is a right angle, then the lines are said to be perpendicular

Perpendicular bisector: If a line drawn perpendicular to other line segment and divides it exactly into two equal parts, it is said to be a perpendicular bisector.

EXERCISE 5.5

1. Which of the following are models for perpendicular lines:

(a) The adjacent edges of a table top.

Ans: The adjacent edges of a table top are perpendicular to each other

(b) The lines of a railway track.

Ans: The lines of a railway track are parallel to each other.

(c) The line segments forming the letter ‘L’.

Ans: The line segments forming the letter ‘L’ are perpendicular to each other

(d) The letter V.

Ans: The sides of letter V are inclined forming an acute angle.

Therefore (a) and (c) are models for perpendicular lines.

2. Let be the perpendicular to the line segment . Let and intersect in the

point A. What is the measure of ∠PAY?

From the figure it is clear that the measure of ∠PAY is 90⁰.

3. There are two set squares in your box. What are the measures of the angles that are

formed at their corners? Do they have any angle measure that is common?

Sol: The measure of angles in one set square are 45⁰, 45⁰ and 90⁰

The other set square has a measure of angles 30⁰, 60⁰ and 90⁰

Yes, the angle of measure 90⁰ is common in between them.

4. Study the diagram. The line l is perpendicular to line m

(a) Is CE = EG? Ans: Yes, since, CE = 2 units and EG = 2 units respectively. (b) Does PE bisect CG? Ans: Yes. Since, CE = EG as both are of 2 units. Hence PE bisect CG. (c) Identify any two line segments for which PE is the perpendicular bisector. Ans: and are the line segments for which PE is the perpendicular bisector. (d) Are these true? (i) AC > FG. Ans: True. Since AC = 2 units and FG = 1 unit. (ii) CD = GH. Ans: True, because both are of 1 unit. (iii) BC < EH. Ans: True. Since, BC = 1 unit & EH = 3 units. ∴ BC < EH

TRIANGLES

Triangles and its types:

Triangle is a polygon with three sides. It is the polygon with the least number of sides. Every

triangle is of different size and shape. We classify them on the basis of their sides and angles.

1. Classification on the basis of sides.

i) Scalene: A scalene triangle has all side lengths of different measures. All the interior angles are also different.

ii) Isosceles: In an isosceles triangle, the lengths of two of the three sides are equal. In other words, an isosceles triangle has two equal sides and two equal angles.

iii) Equilateral: In an equilateral triangle, all the lengths of the sides and all the interior angles are equal.

2. Classification on the basis of Angles

i. Acute Angled Triangle: If all the angles are less than 90° then this is called the acute-angled triangle.

ii. Right Angled Triangle: If one of the angles is 90°then it is called the right-angled triangle.

iii. Obtuse-angled Triangle: If one of the angles of the triangle is obtuse angle then it is called obtuse angled triangle.

Exercise 5.6

1. Name the types of following triangles:

(a) Triangle with lengths of sides 7 cm, 8 cm and 9 cm - Scalene triangle.

(b) ∆ABC with AB = 8.7 cm, AC = 7 cm and BC = 6 cm - Scalene triangle

(c) ∆PQR such that PQ = QR = PR = 5 cm - Equilateral triangle

(d) ∆DEF with ∠D = 90° - Right angled triangle

(e) ∆XYZ with ∠Y = 90° and XY = YZ - Right angled Isosceles triangle

(f) ∆LMN with ∠L = 30°, ∠M = 70° and ∠N = 80° - Acute angled triangle

2. Match the following:

Measures of Triangle Type of Triangle

Answers

(i) 3 sides of equal length (ii) 2 sides of equal length (iii) All sides are of different length (iv) 3 acute angles (v) 1 right angle (vi) 1 obtuse angle (vii) 1 right angle with two sides of equal

length

(a) Scalene (b) Isosceles right angled (c) Obtuse angled (d) Right angled (e) Equilateral (f) Acute angled (g) Isosceles

(i) e (ii) g (iii) a (iv) f (v) d (vi) c (vii) b

3. Name each of the following triangles in two different ways: (you may judge the nature of the

angle by observation)

Sol: (i) Acute angled and isosceles triangle.

(ii) Right angled and scalene triangle.

(iii) Obtuse angled and isosceles triangle.

(iv) Right angled and isosceles triangle.

(v) Equilateral and acute angled triangle.

(vi) Obtuse angled and scalene triangle.

4. Try to construct triangles using match sticks. Some are shown here. Can you make a triangle

with

(a) 3 matchsticks - By using three match sticks we may make an equilateral triangle.

(b) 4 matchsticks - By using 4 match sticks we cannot make a triangle.

(c) 5 matchsticks - By using 5 match sticks we may make an isosceles triangle.

(d) 6 matchsticks - By using 6 match sticks we may make an equilateral triangle.

QUADRILATERALS

Quadrilaterals: A plane figure with four sides is known as quadrilateral.

(v)

Exercise 5.7

1. Say True or False:

(a) Each angle of a rectangle is a right angle. (True)

(b) The opposite sides of a rectangle are equal in length. (True)

(c) The diagonals of a square are perpendicular to one another. (True)

(d) All the sides of a rhombus are of equal length. (True)

(e) All the sides of a parallelogram are of equal length. (False)

(f) The opposite sides of a trapezium are parallel. (False)

2. Give reasons for the following:

(a) A square can be thought of as a special rectangle.

Only opposite sides of the rectangle are of same length whereas in square all the interior

angles are of 90⁰ and all the sides of the square are of same length. Hence, a rectangle with

all sides equal becomes a square. Therefore square is a special rectangle.

(b) A rectangle can be thought of as a special parallelogram

In both parallelogram and rectangle opposite sides are parallel and equal. The interior angles

of the rectangle are of same measure i.e 90⁰. Hence, a parallelogram with each angle as right

angle becomes a square. Therefore a rectangle is a special parallelogram.

(c) A square can be thought of as a special rhombus

All sides of a rhombus and square are equal but in case of square all interior angles are of

90⁰. A rhombus with each angle as right angle becomes a square. Therefore a square is a

special rhombus

(d) Squares, rectangles, parallelograms are all quadrilaterals.

Since, all are closed figures with 4 line segments. Hence all are quadrilaterals

(e) Square is also a parallelogram.

Opposite sides of a parallelogram are equal and parallel whereas in a square opposite sides

are parallel and all 4 sides are of same length. Therefore a square is a special parallelogram.

3. A figure is said to be regular if its sides are equal in length and angles are equal in measure.

Can you identify the regular quadrilateral?

Sol: Square is a regular quadrilateral because all the interior angles are of 90⁰ and all sides are

of same length.

POLYGONS

Polygons: Any closed figure made up of three or more line segments is called Polygon.

Exercise 5.8

1. Examine whether the following are polygons. If anyone among them is not, say why?

Sol: (i) It is not a closed figure. Hence, it is not a polygon.

(ii) It is a polygon made of six sides

(iii) No it is not a polygon because it is not made of line segments.

(iv) It is not a polygon as it is not made of line segments.

2. Name each polygon. Make two more examples of each of these

(a) Quadrilateral

(b) Triangle

(c) Pentagon

(d) Octagon

3. Draw a rough sketch of a regular hexagon. Connecting any three of its vertices, draw a

triangle. Identify the type of the triangle you have drawn.

Sol: Equilateral triangle.

4. Draw a rough sketch of a regular octagon. Draw a rectangle by joining exactly four of the

vertices of the octagon.

Sol: The below figure is a regular octagon in which a rectangle is drawn.

5. A diagonal is a line segment that joins any two vertices of the polygon and is not a side of the

polygon. Draw a rough sketch of a pentagon and draw its diagonals.

Sol: From the figure we may find AC, AD, BD, BE and CE are the diagonals.

THREE-DIMENSIONAL SHAPES

Three-dimensional Shapes: The solid shapes having three dimensions are called 3D shapes.

Some of the 3D shapes around us

Faces, Edges and Vertices

• All the flat surfaces of the solid shape are called the Faces of that figure.

• The line segment where the two faces meet with each other is called Edge.

• The point where the two edges meet with each other is called Vertex.

Exercise 5.9

1. Match the following. Give two new examples of each shape.

(a) An ice cream cone and birthday cap are examples of cone.

(b) Cricket ball and tennis ball are examples of sphere. (c) A road roller and lawn roller are examples of cylinder.

(d) A book and a brick are examples of cuboid.

(e) A diamond and Egypt pyramids are examples of pyramid.

2. What shape is

(a) Your instrument box? (b) A brick? (c) A match box? (d) A road-roller? (e) A sweet laddu?

Ans (a): Cuboid. Ans(b): Cuboid. Ans(c): Cuboid. Ans(d): Cylinder. Ans(e): Sphere.

Reference: OAKS VIDEO LESSON

Prepared by Mrs. Shazida Begum (Boys’ section IISJ)