NCERT Class 9 Mathematics Textbook for Blind and Visually Impaired Students made Screen Readable by Professor T K bansal.

Let us begin our journey by recalling that a variable is denoted by a symbol that can take any real value. We use the letters x, y, z, etc. to denote variables. Notice that 2x, 3x, − x, (− 1/2) x are algebraic expressions. All these expressions are of the form (a constant) × x. Now suppose we want to write an expression which is (a constant) × (a variable) and we do not know what the constant is. In such cases, we write the constant as letters a, b, c, etc. So the expression will be a × x, say.

However, there is a difference between a letter denoting a constant and a letter denoting a variable. The values of a constants remains the same throughout a particular situation, that is, the values of the constant does not change in a given problem, but the value of a variable can keep changing.

Now, let us consider a square of side 3 units (see Figure 2.1).

Note : For the sake of convenience, throughout this chapter, the captions of the figures are given above the figures themselves and the alt text is provided with each and every figure. I hope you will enjoy this lesson, happy learning!

Figure 2.1

What is its perimeter?

We know that the perimeter of a square is the sum of the lengths of its four sides. Here, each side is 3 units. So, its perimeter is 4 × 3, i.e., 12 units. What will be the perimeter if each side of the square is 10 units? The perimeter is 4 × 10, i.e., 40 units. In case the length of each side is x units (see Figure 2.2), the perimeter is given by 4x units. So, as the length of the side varies, the perimeter varies.

Figure 2.2 :

Can we find the area of the square PQRS? It is \(x\ ×\ x\ =\ x^2\) square units. \(X^2\) is an algebraic expression. We are familiar with other algebraic expressions like \(2x;\ x^2\ +\ 2x;\ x^3\ −\ x^2\ +\ 4x\ +\ 7.\) Note that, all the algebraic expressions we have considered so far have only whole numbers as the exponents of one variable only. Expressions of this form are called polynomials in one variable.

In the examples above, the variable is x. For instance, \(x^3\ −\ x^2\ +\ 4x\ +\ 7\) is a polynomial in x.

Similarly, \(3y^2\ +\ 5y\) is a polynomial in the variable y and

\(t^2\ +\ 4\) is a polynomial in the variable t.

In the polynomial \(x^2\ +\ 2x,\) the expressions x^2 and 2x are called the terms of the polynomial. Similarly, the polynomial \(3y^2\ +\ 5y\ +\ 7\) has three terms, namely, 3y^2, 5y and 7. Can you write the terms of the polynomial \(−x^3\ +\ 4x^2\ +\ 7x\ −\ 2?\) This polynomial has 4 terms, namely, \(−x^3,\ 4x^2,\ 7x,\ −2.\)

Each term of a polynomial has a coefficient. So, in \(−x^3\ +\ 4x^2\ +\ 7x\ −\ 2,\) the coefficient of x^3 is −1, the coefficient of x^2 is 4, the coefficient of x is 7 and −2 is the coefficient of x^0. (Remember, x^0 = 1). Do you know the coefficient of x in \(x^2\ −\ x\ +\ 7?\) It is −1.

2 is also a polynomial. In fact, 2, −5, 7, etc. are examples of constant polynomials.

The constant polynomial, 0 is called the zero polynomial. This plays a very important role in the collection of all polynomials, as we will see it in higher classes.

Now, consider algebraic expressions such as \(x\ +\ 1/x;\ √x\ +\ 3;\ \sqrt{3}{y}\ +\ y^2.\) Do you know that we can write x + 1/x = x + x^−1? Here, the exponent of the second term, i.e., x^−1 is −1, which is not a whole number. So, this algebraic expression is not a polynomial.

Again, √x + 3 can be written as x^½ + 3 . Here the exponent of x is ½, which is not a whole number. So, is √x + 3 a polynomial? No, it is not. What about cube root y + y^2? It is also not a polynomial (Why?).

If the variable in a polynomial is x, we may denote the polynomial by p(x), or q(x), or r(x), etc.

So, for example, we may write :
\[p(x)\ =\ 2x^2\ +\ 5x\ −\ 3\]
\[q(x)\ =\ x^3\ −\ 1\]
\[r(y)\ =\ y^3\ +\ y\ +\ 1\]
\[s(u)\ =\ 2\ −\ u\ −\ u^2\ +\ 6u^5\]

A polynomial can have any (finite) number of terms. For instance, \(x^{150}\ +\ x^{149}\ +\ ...\ +\ x^2\ +\ x\ +\ 1\) is a polynomial with 151 terms.

Consider the following polynomials:
\[2x,\]
\[2,\]
\[5x^3,\]
\[−5x^2,\]
\[y\]
and
\[u^4.\]

Don’t we observe that each of these polynomials has only one term? Polynomials having only one term are called monomials (‘ mono ’ means ‘one’).

Now observe each of the following polynomials:
\[p(x)\ =\ x\ +\ 1,\]
\[q(x)\ =\ x^2\ −\ x,\]
\[r(y)\ =\ y^{30}\ +\ 1,\]
\[t(u)\ =\ u^{43}\ −\ u^2\]

How many terms are there in each of these polynomials? Each of these polynomials has only two terms. Polynomials having only two terms are called binomials (‘bi’ means ‘two’).

Similarly, polynomials having only three terms are called trinomials (‘tri’ means ‘three’). Some examples of trinomials are
\[p(x)\ =\ x\ +\ x^2\ +\ π,\]
\[q(x)\ =\ √2\ +\ x\ −\ x^2,\]
\[r(u)\ =\ u\ +\ u^2\ −\ 2,\]
\[t(y)\ =\ y^4\ +\ y\ +\ 5.\]

Now, look at the polynomial
\[p(x)\ =\ 3x^7\ −\ 4x^6\ +\ x\ +\ 9.\]
What is the term with the highest power of x ? It is 3x^7. The exponent of x in this term is 7. Similarly, in the polynomial
\[q(y)\ =\ 5y^6\ −\ 4y^2\ −\ 6,\]
the term with the highest power of y is 5y^6 and the exponent of y in this term is 6.

We call the highest power of the variable in a polynomial as the degree of the polynomial. So, the degree of the polynomial \(3x^7\ −\ 4x^6\ +\ x\ +\ 9\) is 7 and the degree of the polynomial \(5y^6\ −\ 4y^2\ −\ 6\) is 6. The degree of a non-zero constant polynomial is zero.

Example 1

Find the degree of each of the polynomials given below:
\[(i)\ x^5\ −\ x^4\ +\ 3\]
\[(ii)\ 2\ −\ y^2\ −\ y^3\ +\ 2y^8\]
\[(iii)\ 2\]

Solution :

(i) The highest power of the variable is 5. So, the degree of the polynomial is 5.

(ii) The highest power of the variable is 8. So, the degree of the polynomial is 8.

(iii) The only term here is 2 which can be written as 2x^0. So the exponent of x is 0. Therefore, the degree of the polynomial is 0.

Now observe the following polynomials:
\[p(x)\ =\ 4x\ +\ 5,\]
\[q(y)\ =\ 2y,\]
\[r(t)\ =\ t\ +\ √2\]
and
\[s(u)\ =\ 3\ −\ u.\]

Do you see anything common among all of them? The degree of each of these polynomials is one.

A polynomial of degree one is called a linear polynomial.

Some more linear polynomials in one variable are
\[2x\ −\ 1\]
\[√2\ y\ +\ 1\]
\[2\ −\ u.\]

Now, try and find a linear polynomial in x with 3 terms? You would not be able to find it because a linear polynomial in x can have at most two terms. So, any linear polynomial in x will be of the form a x + b, where a and b are constants and a ≠ 0 (why?). Similarly, a y + b is a linear polynomial in y.

Now consider the polynomials :
\[2x^2\ +\ 5\]
\[5x^2\ +\ 3x\ +\ π\]
\[x^2\]
\[x^2\ +\ 2/5\ x\]

Do you agree that they are all of degree two?

A polynomial of degree two is called a quadratic polynomial.

Some more examples of a quadratic polynomial are
\[5\ −\ y^2\]
\[4y\ +\ 5y^2\]
\[6\ −\ y\ −\ y^2.\]

Can we write a quadratic polynomial in one variable with four different terms? We will find that a quadratic polynomial in one variable will have at most 3 terms. If we list a few more quadratic polynomials, we will find that any quadratic polynomial in x is of the form \(ax^2\ +\ bx\ +\ c,\ a\ ≠\ 0\) and a, b, c are constants. Similarly, quadratic polynomial in y will be of the form \(ay^2\ +\ by\ +\ c,\) provided a ≠ 0 and a, b, c are constants.

We call a polynomial of degree three a cubic polynomial.

Some examples of cubic polynomial in x are
\[4x^3\]
\[2x^3\ +\ 1\]
\[5x^3\ +\ x^2
\]6x^3\ −\ x\]
\[6\ −\ x^3\]
\[2x^3\ +\ 4x^2\ +\ 6x\ +\ 7.\]

How many terms do we think a cubic polynomial in one variable can have? It can have at most 4 terms. Cubic polynomial may be written in the form
\[ax^3\ +\ bx^2\ +\ cx\ +\ d,\]
where a ≠ 0 and a, b, c and d are constants.

Now, that we have seen what a polynomial of degree 1, degree 2, or degree 3 looks like, can you write down a polynomial in one variable of degree n for any natural number n? A polynomial in one variable x of degree n is an expression of the form

\[a_nx^n\ +\ a_{n−1}x^{n−1}\ +\ . . . +\ a_1x\ +\ a_0\]

where \(a_0,\ a_1,\ a_2,\ .\ .\ .,\ a_n\) are constants and \(a_n\ ≠\ 0.\)

In particular, if
\[a_0\ =\ a_1\ =\ a_2\ =\ a_3\ =\ .\ .\ .\ =\ a_n\ =\ 0\]
(all the constants are zero), we get the zero polynomial, which is denoted by 0. What is the degree of the zero polynomial? The degree of the zero polynomial is not defined.

So far we have dealt with polynomials in one variable only. We can also have polynomials in more than one variable. For example,

\[x^2\ +\ y^2\ +\ xyz\]
(where variables are x, y and z) is a polynomial in three variables.

Similarly
\[p^2\ +\ q^{10}\ +\ r\]
(where the variables are p, q and r),

\[u^3\ +\ v^2\]
(where the variables are u and v),

are polynomials in three and two variables, respectively.

We’ll be studying such polynomials in detail later.

EXERCISE 2.1

Note : For sake of convenience the answer to each question is given just after the question itself. The letters Q represents the question and A represents the corresponding answer.

Q1. Which of the following expressions are polynomials in one variable and which are not? State reasons for your answer.
\[(i)\ 4x^2\ −\ 3x\ +\ 7\]
\[(ii)\ y^2\ +\ √2\]
\[(iii)\ 3√t\ +\ t\ √2\]
\[(iv)\ y\ +\ 2/y\]
\[(v)\ x^{10}\ +\ y^3\ +\ t^{50}\]

A1.
(i) is a polynomial in one variable,
(ii) is a polynomial in one variable,
(iii) is not a polynomial, because exponent of the variable is not a whole number,
(iv) is not a polynomial, because exponent of the variable is not a whole number,
(v) is a polynomial in three variables,

Q2. Write the coefficients of x^2 in each of the following:
\[(i)\ 2\ +\ x^2\ +\ x\]
\[(ii)\ 2\ −\ x^2\ +\ x^3\]
\[(iii)\ (π/2)×x^2\ +\ x\]
\[(iv)\ √2x\ −\ 1\]

A2.
(i) 1
(ii) − 1
(iii) π/2
(iv) 0.

Q3. Give one example each of a binomial of degree 35, and of a monomial of degree 100.

A3. 3x^35 − 4; square root2 y^100
(You can write many more polynomials with different coefficients.)

Q4. Write the degree of each of the following polynomials:
\[(i)\ 5x^3\ +\ 4x^2\ +\ 7x\]
\[(ii)\ 4\ −\ y^2\]
\[(iii)\ 5t\ −\ √7\]
\[(iv)\ 3\]

A4.
(i) 3
(ii) 2
(iii) 1
(iv) 0

Q5. Classify the following as linear, quadratic and cubic polynomials:
\[(i)\ x^2\ +\ x\]
\[(ii)\ x\ −\ x^3\]
\[(iii)\ y\ +\ y^2\ +\ 4\]
\[(iv)\ 1\ +\ x\]
\[(v)\ 3t\]
\[(vi)\ r^2\]
\[(vii)\ 7x^3\]

A5.
(i) quadratic
(ii) cubic
(iii) quadratic
(iv) linear
(v) linear
(vi) quadratic
(vii) cubic