In our daytoday life, we come across various shapes like curves, lines, triangles and various shapes.
We organize them into line segments, angles, triangles, polygons, and circles.
Here we see that line segment AB is longer than CD.
But we can’t tell how much longer so other methods were deduced.
Here, we have to compare AB and CD.
We will use tracing paper this time.
Trace line segment CD onto the paper and place it over the line segment on AB.
Now, we will be able to decide which is longer. This method depends on the accuracy in tracing the line segment.
But if we want to compare with another line segment you have to trace another line segment.
You cannot trace every time you want to compare.
So, yet another method was used.
Suppose the length is 5 cm, we may write,
Length AB = 5 cm or more simply as
AB = 5 cm.
Open the divider.
Place the endpoint of one of its arms at A and the endpoint of the second arm at B.
When two rays originate from a common point then the turn between two rays around the common point or vertex is called the angle between the two rays.
The two rays joining to form an angle are called arms of an angle and the point at which two rays meet to form an angle is called the vertex of the angle.
In the above figure, two rays and are the arm of an angle which meets at common initial point Q (vertex) and form an ∠PQR.
The measure of the angle PQR is written as ∠PQR but instead of writing this we can simply write it as ∠PQR.
Study the following positions:
You stand facing north.
By a ‘rightangleturn’ clockwise, you now face east.By another ‘rightangleturn’ you finally face south.
The last example was helpful to compare angles with a right angle. We classified the angles as acute, obtuse or reflex.
It cannot be useful if we have to find which one among the two obtuse angles is greater. So in order to be more precise in comparison, we need to ‘measure’ the angles.
We can do it with a ‘protractor’.
The curved edge is divided into 180 equal parts. Each part is equal to a ‘degree’.
We call our measure, ‘degree measure’.
One complete revolution is divided into 360 equal parts. Each part is a degree.
Let us see how to measure the given angle using protractor.
Example:
Put the protractor over the angle and you will be get the accurate measure of angle.
Here, the measure of angle is 60°.
If a line AB is perpendicular to CD, we write AB ⊥ CD.
Triangles can be classified on the basis of
On the basis of side lengths, the triangles are classified into the following types:
Example: Draw a rough sketch of a quadrilateral PQRS.
Draw its diagonals.
Name them.
Is the meeting point of the diagonals in the interior or exterior of the quadrilateral?
Answer:
The two diagonals are PR and QS.Diagonal PR and diagonal QS meet at point T which is in the interior of the quadrilateral PQRS.
We may classify polygons according to the number of their sides.
Let us see some of them:
Number of sides: 3
Name of polygon: Triangle
Illustration:
Number of sides: 4
Name of polygon: Quadrilateral
Illustration:
Number of sides: 5
Name of polygon: Pentagon
Illustration:
Number of sides: 6
Name of polygon: Hexagon
Illustration:
And the list goes on,
For 7 sides its heptagon
For 8 sides its octagon
For 9 sides its nonagon
For 10 sides its decagon...
What is the face of any surface?
The flat surfaces of any solid are called faces.
What are edges?
Line segments common to intersecting faces of a polyhedron are known as its edges. Line segments that form the solid are called edges.
What are vertices?
Points of intersection of edges of a polyhedron are known as its vertices. Corners of the solid are its vertices.
Number of faces, edges, vertices of Cube have been shown below.
Number of faces, edges, vertices of Cuboid have been shown below.
Number of faces, edges, vertices of Pyramid have been shown below.
134 videos325 docs42 tests

1. What is the importance of understanding elementary shapes? 
2. How do we measure line segments? 
3. What are perpendicular lines? 
4. What are threedimensional shapes? 
5. How do we classify triangles? 

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