# NCERT Class 8 Maths Algebraic Expressions and Identities

The chapter 9 begins with an introduction to expressions and representation of expression on a number line.Related terms such as Factors, Coefficients, Monomials, Binomials and Polynomials, Like and Unlike Terms are briefly explained.Then the addition and subtraction of algebraic expressions are discussed in detail.After this the multiplication of algebraic expressions is introduced and various cases under it such as multiplying a monomial by a monomial , by a Polynomial etc is dealt separately.The last section of the chapter deals with standard identities and their application to algebraic expressions.

## Chapter 9 Ex.9.1 Question 1

Identify the terms, their coefficients for each of the following expressions.

(i) $$\quad 5xy{z^2} - 3zy$$

(ii) $$\quad 1 + x + {x^2}$$

(iii) $$\quad 4{x^2}{y^2} - 4{x^2}{y^2}{z^2} + {z^2}$$

(iv) $$\quad 3 - pq + qr - rp$$

(v) \begin{align}\quad \frac{x}{2} + \frac{y}{2} - xy\end{align}

(vi) $$\quad 0.3a - 0.6ab + 0.5b$$

### Solution

What is known?

Expressions

What is unknown?

Terms and their coefficients

Reasoning:

The numerical factor of a term is called its numerical coefficient or simply coefficient.

Steps:

The terms and the respective coefficients of the given expressions are as follows.

 - Terms Coefficients (i) \begin{align}&{{5}}xy{z^{\rm{2}}}\\&{{ -3}}zy\end{align} \begin{align}5\\- 3\end{align} (ii) \begin{align}&1\\&x\\&{x^2}\end{align} \begin{align}&1\\&1\\&1\end{align} (iii) \begin{align}&4{x^2}{y^2}\\- &4{x^2}{y^2}{z^2}\\&{z^2}\end{align} \begin{align}&4\\-& 4\\&1\end{align} (iv) \begin{align}&{3}\\ - &pq\\&qr\\ - &rp\end{align} \begin{align}&3\\- &1\\&1\\- &1\end{align} (v) \begin{align}&\frac{x}{2}\\&\frac{y}{2}\\- &xy\end{align} \begin{align}&\frac{1}{2}\\&\frac{1}{2}\\- &1\end{align} (vi) \begin{align}&0.3a\\- &0.6ab\\&0.5b\end{align} \begin{align}&0.3\\- &0.6\\&0.5\end{align}

## Chapter 9 Ex.9.1 Question 2

Classify the following polynomials as monomials, binomials, trinomials. Which polynomials do not fit in any of these three categories?

\begin{align}&x + y,\\ &1000,\\ & x + {x^2} + {x^3} + {x^4},\\ & 7 + y + 5x,\\ & 2y - 3{y^2},\\ & 2y - 3{y^2} + 4{y^3},\\ &5x - 4y + 3xy,\\ & 4z - 15{z^2},\\ & ab + bc + cd + da,\\ & pqr,\\ & {p^2}q + p{q^2},\\ & 2p + 2q\end{align}

### Solution

What is known?

Expression

What is unknown?

The degree of the expression.

Reasoning:

1. Expression that contains only one term is called a monomial.

2. Expression that contains two terms is called a binomial.

3. Expression containing three terms is a trinomial and so on.

4. An expression containing, one or more terms with non-zero coefficient (with variables having non-negative integers as exponents) is called a polynomial.

5. A polynomial may contain any number of terms, one or more than one.

Steps:

The given expressions are classified as

Monomials: $$1000,\; pqr$$

Binomials:

\begin{align} &x + y,\;2y - 3{y^2},\\ & 4z - 15{z^2},\\ & {p^2}q + p{q^2},\\ & 2p + 2q \end{align}

Trinomials:

\begin{align} &7 + y + 5x,\\ & 2y - 3{y^2} + 4{y^3},\\ & 5x - 4y + 3xy \end{align}

Polynomials that do not fit in any of these categories are

\begin{align} x + {x^2} + {x^3} + {x^4},\\ab + bc + cd + da \end{align}

## Chapter 9 Ex.9.1 Question 3

(i)$$\quad ab\text{ }-\text{ }bc,\; bc\text{ }-\text{ }ca,\; ca\text{ }-\text{ }ab$$

(ii)\begin{align} \begin{Bmatrix} a - b + ab,\\ b- c + bc ,\\ c - a + ac \end{Bmatrix} \end{align}

(iii)\begin{align} \begin{Bmatrix} \quad 2{{p}^{2}}{{q}^{2}}-3pq+4,\\ 5+7pq -3{{p}^{2}}{{q}^{2}} \end{Bmatrix} \end{align}

(iv)\begin{align} \begin{Bmatrix} {l^2} + {m^2},\\ {m^2} + {n^2},{n^2} + {l^2},\\ 2lm + 2mn + 2nl \end{Bmatrix} \end{align}

### Solution

What is known?

Expressions

What is unknown?

Reasoning:

Addition will take place between like terms.

Steps:

The given expressions written in separate rows, with like terms one below the other

and then the addition of these expressions are as follows.

(i)

\frac{\begin{align}\,\,&\,\,\,\,\,ab - bc\\&+\qquad bc - ca\\&+ \, - ab\; + ca\end{align}}{0}

Thus, the sum of the given expressions is $$0.$$

(ii)

\frac{\begin{align} & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,a-b+ab \\& +\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,b\,\,\,\,\,\,\,\,\,\,\,\,\,\,-c+bc \\& +\,\,\,\,\,\,\,\,-a\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,+c\,\,\,\,\,\,\,\,\,\,\,\,+ac \\\end{align}}{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,ab+\,\,\,\,\,\,\,\,bc\,\,\,\,\,\,\,\,+ac}

Thus, the sum of the given expressions is $$ab + bc + ac.$$

(iii)

\frac{\begin{align}2{p^2}{q^2} \;-\; 3pq\; + \;4\\+ \,\,\, - 3{p^2}{q^2}\;+\; 7pq \;+\; 5\end{align}}{{\,\,\,\,\,\, - {p^2}{q^2}\;\; + \;\;4pq\;\; +\;\; 9}}

Thus, the sum of the given expressions is $$- {p^2}{q^2} + 4pq + 9.$$

(iv)

\frac{\begin{align}& \,\,\,\,\,\,{{l}^{2}}+{{m}^{2}} \\& +\,\,\,\,+{{m}^{2}}+{{n}^{2}} \\& +{{l}^{2}}\qquad \; \;+{{n}^{2}} \\& + \qquad \qquad \quad \; 2lm+2mn+2nl \\\end{align}}{\begin{align} 2{{l}^{2}}& +2{{m}^{2}} +2{{n}^{2}} \\ &+2lm+2mn+2nl \end{align}}

Thus, the sum of the given expressions is $$2$$($${{l}^{2}}+{{m}^{2}}+{{n}^{2}}+lm+mn+nl$$)

## Chapter 9 Ex.9.1 Question 4

(a) Subtract $$4a - 7ab + 3b + 12$$ from

$$12a - 9ab + 5b - 3$$

(b) Subtract $$3xy + 5yz - 7zx$$ from

$$5xy - 2yz - 2zx + 10xyz$$

(c) Subtract $$\begin{Bmatrix}4{p^2}q - 3pq + 5p{q^2} \\ - 8p + 7q - 10\end{Bmatrix}$$ from

$$\begin{Bmatrix}18 - 3p - 11q +\\ 5pq - 2p{q^2} + 5{p^2}q\end{Bmatrix}$$

### Solution

What is known?

Expressions

What is unknown?

Subtraction

Reasoning:

Subtraction will take place between like terms.

Steps:

The given expressions in separate rows, with like terms one below the other and

then the subtraction of these expressions is as follows.

(a)

\dfrac{\begin{align}&12a\,\,\,\,\,\, - 9ab\,\,\,\,\,\, + 5b\,\,\,\,\,\, - 3\\&4a\,\,\,\,\,\, - 7ab\,\,\,\,\,\, + 3b\,\,\,\,\,\,\, + 12\\&\left(-\right)\,\,\,\,\,\,\,\,\left(+\right)\,\,\,\,\,\,\,\,\,\,\,\left(-\right)\,\,\,\,\,\,\,\,\,\,\left(-\right)\end{align}}{{\!\!\!\!\!\!\!\!8a - 2ab\,\,\,\, + 2b - 15}}

(b)

\dfrac{\begin{align}&5xy - 2yz - 2zx + 10xyz\\&3xy + 5yz - 7zx\\&\left(-\right)\,\,\,\,\,\,\,\,\left(-\right)\,\,\,\,\,\,\left(+\right)\end{align}}{{\,\,\,\,\,\,\,2xy - 7yz + 5zx + 10xyz}}

(c)

\dfrac{\begin{align}18 - 3p\! - 11q \!+\! 5pq \!- 2p{q^2} \!+ \!5{p^2}q \\ - 10 - 8p +\! 7p\! - \!3pq +\! 5p{q^2} + \!4{p^2}q\\\left(+\right)\,\,\,\,\left(+\right)\,\,\,\,\left(-\right)\,\,\,\,\,\,\left(+\right)\,\,\,\,\left(-\right)\,\,\,\,\,\left(-\right)\,\,\,\,\;\end{align}}{28\!+\!5p \!- \!18q \!+\! 8pq \!- \!7p q^2\! +\! p^2 q}

Algebraic Expressions and Identities | NCERT Solutions
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