NCERT Textbook- Circles Class 10 Notes | EduRev

Mathematics (Maths) Class 10

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Class 10 : NCERT Textbook- Circles Class 10 Notes | EduRev

 Page 1


206 MATHEMA TICS
10
10.1 Introduction
You have studied in Class IX that a circle is a collection of all points in a plane
which are at a constant distance (radius) from a fixed point (centre). You have
also studied various terms related to a circle like chord, segment, sector, arc etc.
Let us now examine the different situations that can arise when a circle and a line
are given in a plane.
So, let us consider a circle and a line PQ. There can be three possibilities given
in Fig. 10.1 below:
Fig. 10.1
In Fig. 10.1 (i), the line PQ and the circle have no common point. In this case,
PQ is called a non-intersecting line with respect to the circle. In Fig. 10.1 (ii), there
are two common points A and B that the line PQ and the circle have. In this case, we
call the line PQ a secant of the circle. In Fig. 10.1 (iii), there is only one point A which
is common to the line PQ and the circle. In this case, the line is called a tangent to the
circle.
CIRCLES
Page 2


206 MATHEMA TICS
10
10.1 Introduction
You have studied in Class IX that a circle is a collection of all points in a plane
which are at a constant distance (radius) from a fixed point (centre). You have
also studied various terms related to a circle like chord, segment, sector, arc etc.
Let us now examine the different situations that can arise when a circle and a line
are given in a plane.
So, let us consider a circle and a line PQ. There can be three possibilities given
in Fig. 10.1 below:
Fig. 10.1
In Fig. 10.1 (i), the line PQ and the circle have no common point. In this case,
PQ is called a non-intersecting line with respect to the circle. In Fig. 10.1 (ii), there
are two common points A and B that the line PQ and the circle have. In this case, we
call the line PQ a secant of the circle. In Fig. 10.1 (iii), there is only one point A which
is common to the line PQ and the circle. In this case, the line is called a tangent to the
circle.
CIRCLES
CIRCLES 207
You might have seen a pulley fitted over a well which is used
in taking out water from the well. Look at Fig. 10.2. Here the rope
on both sides of the pulley, if considered as a ray, is like a tangent
to the circle representing the pulley.
Is there any position of the line with respect to the circle
other than the types given above? You can see that there cannot
be any other type of position of the line with respect  to the circle.
In this chapter, we will study about the existence of the tangents
to a circle and also study some of their properties.
10.2 Tangent to a Circle
In the previous section, you have seen that a tangent* to a circle is a line that
intersects the circle at only one point.
To understand the existence of the tangent to a circle at a point, let us perform
the following activities:
Activity 1 : Take a circular wire and attach a straight wire AB at a point P of the
circular wire so that it can rotate about the point P in a plane. Put the system on a table
and gently rotate the wire AB about the point P to get different positions of the straight
wire [see Fig. 10.3(i)].
In various positions, the wire intersects the
circular wire at P and at another point Q
1
 or Q
2
 or
Q
3
, etc. In one position, you will see that it will
intersect the circle at the point P only (see position
A'B' of AB). This shows that a tangent exists at
the point P of the circle. On rotating further, you
can observe that in all other positions of AB, it will
intersect the circle at P and at another point, say R
1
or R
2
 or R
3
,
 
etc. So, you can observe that there is
only one tangent at a point of the circle.
While doing activity above, you must have observed that as the position AB
moves towards the position A' B', the common point, say Q
1
, of the line AB and the
circle gradually comes nearer and nearer to the common point P . Ultimately, it coincides
with the point P in the position A'B' of A''B''. Again note, what happens if ‘AB’ is
rotated rightwards about P? The common point R
3
 gradually comes nearer and nearer
to P and ultimately coincides with P. So, what we see is:
The tangent to a circle is a special case of the secant, when the two end
points of its corresponding chord coincide.
Fig. 10.3 (i)
Fig. 10.2
*The word ‘tangent’ comes from the Latin word ‘tangere’, which means to touch and was
introduced by the Danish mathematician Thomas Fineke in 1583.
Page 3


206 MATHEMA TICS
10
10.1 Introduction
You have studied in Class IX that a circle is a collection of all points in a plane
which are at a constant distance (radius) from a fixed point (centre). You have
also studied various terms related to a circle like chord, segment, sector, arc etc.
Let us now examine the different situations that can arise when a circle and a line
are given in a plane.
So, let us consider a circle and a line PQ. There can be three possibilities given
in Fig. 10.1 below:
Fig. 10.1
In Fig. 10.1 (i), the line PQ and the circle have no common point. In this case,
PQ is called a non-intersecting line with respect to the circle. In Fig. 10.1 (ii), there
are two common points A and B that the line PQ and the circle have. In this case, we
call the line PQ a secant of the circle. In Fig. 10.1 (iii), there is only one point A which
is common to the line PQ and the circle. In this case, the line is called a tangent to the
circle.
CIRCLES
CIRCLES 207
You might have seen a pulley fitted over a well which is used
in taking out water from the well. Look at Fig. 10.2. Here the rope
on both sides of the pulley, if considered as a ray, is like a tangent
to the circle representing the pulley.
Is there any position of the line with respect to the circle
other than the types given above? You can see that there cannot
be any other type of position of the line with respect  to the circle.
In this chapter, we will study about the existence of the tangents
to a circle and also study some of their properties.
10.2 Tangent to a Circle
In the previous section, you have seen that a tangent* to a circle is a line that
intersects the circle at only one point.
To understand the existence of the tangent to a circle at a point, let us perform
the following activities:
Activity 1 : Take a circular wire and attach a straight wire AB at a point P of the
circular wire so that it can rotate about the point P in a plane. Put the system on a table
and gently rotate the wire AB about the point P to get different positions of the straight
wire [see Fig. 10.3(i)].
In various positions, the wire intersects the
circular wire at P and at another point Q
1
 or Q
2
 or
Q
3
, etc. In one position, you will see that it will
intersect the circle at the point P only (see position
A'B' of AB). This shows that a tangent exists at
the point P of the circle. On rotating further, you
can observe that in all other positions of AB, it will
intersect the circle at P and at another point, say R
1
or R
2
 or R
3
,
 
etc. So, you can observe that there is
only one tangent at a point of the circle.
While doing activity above, you must have observed that as the position AB
moves towards the position A' B', the common point, say Q
1
, of the line AB and the
circle gradually comes nearer and nearer to the common point P . Ultimately, it coincides
with the point P in the position A'B' of A''B''. Again note, what happens if ‘AB’ is
rotated rightwards about P? The common point R
3
 gradually comes nearer and nearer
to P and ultimately coincides with P. So, what we see is:
The tangent to a circle is a special case of the secant, when the two end
points of its corresponding chord coincide.
Fig. 10.3 (i)
Fig. 10.2
*The word ‘tangent’ comes from the Latin word ‘tangere’, which means to touch and was
introduced by the Danish mathematician Thomas Fineke in 1583.
208 MATHEMA TICS
Activity 2 : On a paper, draw a circle and a
secant PQ of the circle. Draw various lines
parallel to the secant on both sides of it. You
will find that after some steps, the length of
the chord cut by the lines will gradually
decrease, i.e., the two points of intersection of
the line and the circle are coming closer and
closer [see Fig. 10.3(ii)]. In one case, it
becomes zero on one side of the secant and in
another case, it becomes zero on the other side
of the secant. See the positions P'Q' and P''Q''
of the secant in Fig. 10.3 (ii). These are the
tangents to the circle parallel to the given secant
PQ. This also helps you to see that there cannot
be more than two tangents parallel to a given
secant.
This activity also establishes, what you must have observed, while doing
Activity 1, namely, a tangent is the secant when both of the end points of the
corresponding chord coincide.
The common point of the tangent and the circle is called the point of contact
[the point A in Fig. 10.1 (iii)]and the tangent is said to touch the circle at the
common point.
Now look around you. Have you seen a bicycle
or a cart moving? Look at its wheels. All the spokes
of a wheel are along its radii. Now note the position
of the wheel with respect to its movement on the
ground. Do you see any tangent anywhere?
(See Fig. 10.4). In fact, the wheel moves along a line
which is a tangent to the circle representing the wheel.
Also, notice that in all positions, the radius through
the point of contact with the ground appears to be at
right angles to the tangent (see Fig. 10.4). We shall
now prove this property of the tangent.
Theorem 10.1 : The tangent at any point of a circle is perpendicular to the
radius through the point of contact.
Proof : We are given a circle with centre O and a tangent XY to the circle at a
point P. We need to prove that OP is perpendicular to XY.
Fig. 10.4
Fig. 10.3 (ii)
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