Revision Notes for Class 9 Maths Chapter 2 Polynomials

NCERT Notes for Class 9 Maths Chapter 2 Polynomials

Class 9 Maths Chapter 2 Polynomials Notes

Chapter Name	Polynomials Notes
Class	CBSE Class 9
Textbook Name	Mathematics Class 9
Related Readings	Notes for Class 9 Notes for Class 9 Maths Revision Notes for Polynomials

Polynomials

Algebraic expression

An expression which is the combination of constants and variables and are connected by some or all the operations addition, subtraction, multiplication and division is known as an algebraic expression.
Example: 7 + 9x – 2x²+ 5/6 xy

Constant

Which has fixed numerical value.
Example: 7, -4, 3/4 , n etc.

Variable

A symbol which has no fixed numerical value is known as a variable.
Example: 2x, 5x²

Terms

These are the parts of an algebraic expression which are separated by operations, like addition or subtraction are known as terms.
Example: In the expression 5x³+ 9x²+ 7x – 3, terms are 5x³, 9x², 7x and -3.

Polynomials

An algebraic expression of which variables have non-negative integral powers is called a polynomial.
Example:
(a) 5x² + 7x + 3
(b) 9y³– 7y² + 3y + 7

Coefficient

A coefficient is the numerical value in a term.
Note: If a term has no coefficient, the coefficient is an unwritten 1.
Example: 5x³– 7x² – x + 3

Degree of a polynomial (in one variable)

The highest power of the variable is called the degree of the polynomial.
Example: 5x + 4 is a polynomial in x of degree 1.

Degree of a polynomial in two or more variables

The highest sum of powers of variables is called the degree of the polynomial.
Example: 7x³ + 2x²y² – 3ry + 8

Degree of polynomial = 4 (Sum of the powers of variables x and y )

Types of Polynomials

1. Linear polynomial

A polynomial of degree one is called a linear polynomial.
Example: 2x + 3 is a linear polynomial in x.

2. Quadratic polynomial

A polynomial of degree 2 is called a quadratic polynomial.
Example: 5x² – 7x + 4 is a quadratic polynomial.

3. Cubic polynomial

A polynomial of degree 3 is called a cubic polynomial.
Example: 3x³ + 7x² – 4x + 9 is a cubic polynomial.

4. Biquadratic polynomial

A polynomial of degree 4 is called a biquadratic polynomial.
Example: 7x⁴ – 2x³ + 4x + 9 is a biquadratic polynomial.

Number of Terms in a Polynomial

Categories of the polynomial according to their terms

(i) Monomial

A polynomial which has only one non-zero term is called a monomial.
Example: 7, 4x, (4/5)xy, 7x²y³z⁵, all are monomials.

(ii) Binomial

A polynomial which has only two non-zero terms is called binomial.
Example: 2x + 7, 9x² + 3, 3x²yz + 4x³y³z², all are binomials.

(iii) Trinomial

A polynomial which has only three non-zero terms is called a trinomial.
Example: 5x² + lx + 9, 5xy + 7xy² + 3x³yz, all are trinomials.

(iv) Constant polynomial

A polynomial which has only one term and that is a constant is called a constant polynomial.
Example: (−3/4), 7, 5 all are constant polynomials.
Note: The degree of constant non-zero polynomial is zero.

(v) Zero polynomial

A polynomial which has only one term i.e., 0 is called a zero polynomial.
Note: Degree of a zero polynomial is not defined.

Value of a Polynomial

Value of a polynomial is obtained, when variable of a given polynomial is interchanged or replaced by a constant.

Let p(x) is a polynomial then value of polynomial at x = a is p(a).

Zero or root of a polynomial: A zero or root of a polynomial is the value of that variable for which value of polynomial p(x) becomes zero i.e., p(x) = 0.
Let p(x) be the polynomial and x – a.
If p(a) = 0 then real value a is called zero of a polynomial.

Remainder Theorem

Let p(x) be a polynomial of degree ≥ 1 and a be any real number. If p(x) is divided by the linear polynomial x-a, then the remainder is p(a).

Proof: Let p(x) be any polynomial of degree greater than or equal to 1. When p(x) is divided by x–a, the quotient is q(x) and remainder is r(x).
i.e. p(x) = (x-a) q(x) + r(x)

Since, degree of (x–a) is 1 and the degree of r(x) is less than the degree of (x–a) so the degree of r(x) = 0.
It: means r(x) is a constant, say r.
Therefore, for every value of x, r(x) = r
then p(x) = (x-a) q(x) + r
When x = a, then p(a) = (a – a) q(x) + r ⇒ p(a) = r

Factor Theorem

If p(x) is a polynomial of degree greater than or equal to 1 and a be any real number, then

• x–a is a factor of p(x) i.e., p(x) – (x-a) q(x) which shows x–a is a factor of p(x).
• Since x–a is a factor of p(x)
  p(x) = (x-a) g(x) for same polynomial g(x). In this case, p(a) = (a-a) g (a) = 0

Factorisation of the Polynomial ax²+ bx + c by Splitting the Middle Term

Let p(x) = ax²+ bx + c
and factor of polynomial p(x) = (px + q) and (rx + s)
then ax² + bx + c = (px + q) (rx + s) = prx² + (ps +qr)x+ qs
Comparing the coefficient of x² on both sides
a = pr ...(1)
Comparing the coefficient of x
b = ps + qr …(2)
and comparing the constant terms
c = qs …(3)
which shows that b is the sum of two numbers ps + qr.
Product of two numbers ps × qr =pr × qs = ac
So, for factors ax² + bx + c, we should write b as sum of two numbers whose product is ac.

Example: Factorise 6x² + 17x + 5
Here, b = p + q = 17
and ac = 6 × 5 = 30 (= pq)
then we get factors of 30, 1 × 30, 2 × 15, 3 × 10, 5 × 6,
Among above factors of 30, the sum of 2 and 15 is 17
i.e., p + q = 2 + 15 = 17
∴ 6x² + 17x + 5

= 6x² + (2 + 15)x + 5

= 6x² + 2x + 15x + 5
= 2x(3x + 1) + 5(3x + 1)

= (3x + 1) (2x + 5)

Algebraic Identities