NCERT Class 9 Mathematics Textbook for Blind Students made Screen Readable by Professor T K Bansal.

We have studied the number line in the chapter on ‘Number System’. On the number line, distances from a fixed point are marked in equal units positively in one direction and negatively in the other. The point from which the distances are marked is called the origin. We use the number line to represent the numbers by marking points on a line at equal distances. If one unit distance represents the number ‘1’, then 3 units distance represents the number ‘3’, ‘0’ being at the origin. The point in the positive direction at a distance r from the origin represents the number r. The point in the negative direction at a distance r from the origin represents the number −r. Locations of different numbers on the number line are shown in Fig. 3.5.

Fig. 3.5

Descartes invented the idea of placing two such lines perpendicular to each other on a plane, and locating points on the plane by referring them to these lines. The perpendicular lines may be in any direction such as in Fig.3.6. But, when we choose these two lines to locate a point in a plane in this chapter, one line will be horizontal and the other will be vertical, as in Fig. 3.6(c).

Fig. 3.6 (a and b)

Fig. 3.6 c

These lines are actually obtained as follows : Take two number lines, call them X′X and Y′Y. Place X′X horizontal [as in Fig. 3.7(a)] and write the numbers on it just as written on the number line. We do the same thing with Y′Y except that Y′Y is vertical, not horizontal [Fig. 3.7(b)].

Fig. 3.7 (a)

Fig. 3.7 (b)

Combine both the lines in such a way that the two lines cross each other at their zeroes, or origins (Fig. 3.8). The horizontal line X′X is called the x - axis and the vertical line Y′Y is called the y - axis. The point where X′X and Y′Y cross, is called the origin, and is denoted by the letter O. Since the positive numbers lie on the directions O X and O Y, O X and O Y are called the positive directions of the x - axis and the y - axis, respectively. Similarly, OX′and OY′ are called the negative directions of the x - axis and the y - axis, respectively.

Fig. 3.8

We observe that the axes (plural of the word ‘ axis ’) divide the plane into four parts. These four parts are called the quadrants (one fourth part), numbered I, II, III and IV moving in the anticlockwise direction from O X (see Fig.3.9). So, the plane consisting of the axes and these quadrants is called the Cartesian plane, or the coordinate plane, or the x y-plane. The axes are called the coordinate axes.

Fig. 3.9

Now, let us see why this system is so basic to mathematics, and how it is useful. Consider the following diagram where the axes are drawn on graph paper. Let us see the distances of the points P and Q from the axes. For this, we draw perpendiculars PM on the x - axis and PN on the y - axis. Similarly, we draw perpendiculars QR and QS as shown in Fig. 3.10.

Fig.3.10

We find that
(i) The perpendicular distance of the point P from the y - axis measured along the positive direction of the x - axis is PN = O M = 4 units.
(ii) The perpendicular distance of the point P from the x - axis measured along the positive direction of the y - axis is PM = O N = 3 units.
(iii) The perpendicular distance of the point Q from the y - axis measured along the negative direction of the x - axis is OR = SQ = 6 units.
(iv) The perpendicular distance of the point Q from the x - axis measured along the negative direction of the y - axis is OS = RQ = 2 units.

Now, using these distances, how can we describe the points so that there is no confusion?

We write the coordinates of a point, using the following conventions:

(i) The x - coordinate of a point is its perpendicular distance from the y- axis measured along the x -axis (positive along the positive direction of the x-axis and negative along the negative direction of the x-axis). For the point P, it is + 4 and for Q, it is −6. The x - coordinate is also called the abscissa.

(ii) The y - coordinate of a point is its perpendicular distance from the x - axis measured along the y-axis (positive along the positive direction of the y-axis and negative along the negative direction of the y-axis). For the point P, it is + 3 and for Q, it is −2. The y-coordinate is also called the ordinate.

(iii) In stating the coordinates of a point in the coordinate plane, the x - coordinate comes first, and then the y - coordinate. The two coordinates are separated by a comma (,). We place the coordinates in round brackets.

Hence, the coordinates of P are (4, 3) and the coordinates of Q are ( −6, −2).

Note that the coordinates describe a point in the plane uniquely. (3, 4) is not the same as (4, 3).

Example 1

See Fig. 3.11 and complete the following statements:

Fig. 3.11

(i) The abscissa and the ordinate of the point B are _ _ _ and _ _ _, respectively. Hence, the coordinates of B are (_ _, _ _).
(ii) The x-coordinate and the y-coordinate of the point M are _ _ _ and _ _ _, respectively. Hence, the coordinates of M are (_ _, _ _).
(iii) The x-coordinate and the y-coordinate of the point L are _ _ _ and _ _ _, respectively. Hence, the coordinates of L are (__, ___).
(iv) The x-coordinate and the y-coordinate of the point S are _ _ _ and _ _ _, respectively. Hence, the coordinates of S are (__, __).

Solution :

(i) Since the distance of the point B from the y - axis is 4 units, the x - coordinate or abscissa of the point B is 4. The distance of the point B from the x - axis is 3 units, therefore, the y - coordinate, i.e., the ordinate of the point B is 3.
Hence, the coordinates of the point B are (4, 3).

(ii) The x - coordinate and the y - coordinate of the point M are −3 and 4, respectively.
Hence, the coordinates of the point M are (−3, 4).

(iii) The x - coordinate and the y - coordinate of the point L are −5 and −4, respectively.
Hence, the coordinates of the point L are (−5, −4).

(iv) The x - coordinate and the y- coordinate of the point S are 3 and −4, respectively.
Hence, the coordinates of the point S are (3, −4).

Example 2

Write the coordinates of the points marked on the axes in Fig. 3.12.

Fig. 3.12

Solution :

We can see that :

(i) The point A is at a distance of + 4 units from the y - axis and at a distance of zero from the x - axis. Therefore, the x - coordinate of A is 4 and the y - coordinate is 0.
Hence, the coordinates of A are (4, 0).

(ii) The coordinates of B are (0, 3). Why?

(iii) The coordinates of C are (− 5, 0). Why?

(iv) The coordinates of D are (0, − 4). Why?

(v) The coordinates of E are (2/3, 0). Why?

Since every point on the x - axis has no distance (zero distance) from the x - axis, therefore, the y - coordinate of every point lying on the x - axis is always zero. Thus, the coordinates of any point on the x - axis are of the form (x, 0), where x is the distance of the point from the y - axis. Similarly, the coordinates of any point on the y - axis are of the form (0, y), where y is the distance of the point from the x - axis. Why?

What are the coordinates of the origin O? It has zero distance from both the axes so that both its abscissa and ordinate are 0. Therefore, the coordinates of the origin are (0, 0).

In the examples above, you may have observed the following relationship between the signs of the coordinates of a point and the quadrant of a point in which it lies.

(i) If a point is in the 1st quadrant, then the point will be in the form (+, +), since the 1st quadrant is enclosed by the positive x - axis and the positive y - axis.

(ii) If a point is in the 2nd quadrant, then the point will be in the form (−, +), since the 2nd quadrant is enclosed by the negative x - axis and the positive y - axis.

(iii) If a point is in the 3rd quadrant, then the point will be in the form (−, −), since the 3rd quadrant is enclosed by the negative x - axis and the negative y - axis.

(iv) If a point is in the 4th quadrant, then the point will be in the form (+, −), since the 4th quadrant is enclosed by the positive x - axis and the negative y - axis (see Fig. 3.13).

Fig. 3.13

Remark : The system we have discussed above for describing a point in a plane is only a convention, which is accepted all over the world. The system could also have been, for example, the ordinate first, and the abscissa second. However, the whole world sticks to the system we have described to avoid any confusion.

EXERCISE 3.2

Q1. Write the answer of each of the following questions:
(i) What is the name of horizontal and the vertical lines drawn to determine the position of any point in the Cartesian plane?
(ii) What is the name of each part of the plane formed by these two lines?
(iii) Write the name of the point where these two lines intersect.

A1.
(i) The x - axis and the y - axis
(ii) Quadrants
(iii) The origin

Q2. See Fig.3.14, and write the following:

Fig. 3.14

(i) The coordinates of B.
(ii) The coordinates of C.
(iii) The point identified by the coordinates (-3, −5).
(iv) The point identified by the coordinates (2, −4).
(v) The abscissa of the point D.
(vi) The ordinate of the point H.
(vii) The coordinates of the point L.
(viii) The coordinates of the point M.

A2.
(i) ( −5, 2)
(ii) (5, −5)
(iii) E
(iv) G
(v) 6
(vi) −3
(vii) (0, 5)
(viii) (−3, 0)