4.1 Introduction

In earlier classes, you have studied linear equations in one variable. Can you write down a linear equation in one variable? You may say that $x+1=0, x+\sqrt{2}=0$ and $\sqrt{2} y+\sqrt{3}=0$ are examples of linear equations in one variable. You also know that such equations have a unique (i.e., one and only one) solution. You may also remember how to represent the solution on a number line. In this chapter, the knowledge of linear equations in one variable shall be recalled and extended to that of two variables. You will be considering questions like: Does a linear equation in two variables have a solution? If yes, is it unique? What does the solution look like on the Cartesian plane? You shall also use the concepts you studied in Chapter 3 to answer these questions.

4.2 Linear Equations

Let us first recall what you have studied so far. Consider the following equation:

$$ 2 x+5=0 $$

Its solution, i.e., the root of the equation, is $-\frac{5}{2}$. This can be represented on the number line as shown below:

Fig. 4.1

While solving an equation, you must always keep the following points in mind:

The solution of a linear equation is not affected when:

(i) the same number is added to (or subtracted from) both the sides of the equation.

(ii) you multiply or divide both the sides of the equation by the same non-zero number.

Let us now consider the following situation:

In a One-day International Cricket match between India and Sri Lanka played in Nagpur, two Indian batsmen together scored 176 runs. Express this information in the form of an equation.

Here, you can see that the score of neither of them is known, i.e., there are two unknown quantities. Let us use $x$ and $y$ to denote them. So, the number of runs scored by one of the batsmen is $x$, and the number of runs scored by the other is $y$. We know that

$$ x+y=176 $$

which is the required equation.

This is an example of a linear equation in two variables. It is customary to denote the variables in such equations by $x$ and $y$, but other letters may also be used. Some examples of linear equations in two variables are:

$$ 1.2 s+3 t=5, p+4 q=7, \pi u+5 v=9 \text { and } 3=\sqrt{2} x-7 y . $$

Note that you can put these equations in the form $1.2 s+3 t-5=0$, $p+4 q-7=0, \pi u+5 v-9=0$ and $\sqrt{2} x-7 y-3=0$, respectively.

So, any equation which can be put in the form $a x+b y+c=0$, where $a, b$ and $c$ are real numbers, and $a$ and $b$ are not both zero, is called a linear equation in two variables. This means that you can think of many many such equations.

4.3 Solution of a Linear Equation

You have seen that every linear equation in one variable has a unique solution. What can you say about the solution of a linear equation involving two variables? As there are two variables in the equation, a solution means a pair of values, one for $x$ and one for $y$ which satisfy the given equation. Let us consider the equation $2 x+3 y=12$. Here, $x=3$ and $y=2$ is a solution because when you substitute $x=3$ and $y=2$ in the equation above, you find that

$$ 2 x+3 y=(2 \times 3)+(3 \times 2)=12 $$

This solution is written as an ordered pair $(3,2)$, first writing the value for $x$ and then the value for $y$. Similarly, $(0,4)$ is also a solution for the equation above.

On the other hand, $(1,4)$ is not a solution of $2 x+3 y=12$, because on putting $x=1$ and $y=4$ we get $2 x+3 y=14$, which is not 12 . Note that $(0,4)$ is a solution but $\operatorname{not}(4,0)$.

You have seen at least two solutions for $2 x+3 y=12$, i.e., $(3,2)$ and $(0,4)$. Can you find any other solution? Do you agree that $(6,0)$ is another solution? Verify the same. In fact, we can get many many solutions in the following way. Pick a value of your choice for $x(\operatorname{say} x=2)$ in $2 x+3 y=12$. Then the equation reduces to $4+3 y=12$, which is a linear equation in one variable. On solving this, you get $y=\frac{8}{3}$. So $\left(2, \frac{8}{3}\right)$ is another solution of $2 x+3 y=12$. Similarly, choosing $x=-5$, you find that the equation becomes $-10+3 y=12$. This gives $y=\frac{22}{3}$. So, $\left(-5, \frac{22}{3}\right)$ is another solution of $2 x+3 y=12$. So there is no end to different solutions of a linear equation in two variables. That is, a linear equation in two variables has infinitely many solutions.

Summary

In this chapter, you have studied the following points:

1. An equation of the form $a x+b y+c=0$, where $a, b$ and $c$ are real numbers, such that $a$ and $b$ are not both zero, is called a linear equation in two variables.

2. A linear equation in two variables has infinitely many solutions.

3. Every point on the graph of a linear equation in two variables is a solution of the linear equation. Moreover, every solution of the linear equation is a point on the graph of the linear equation.