# Ex.5.1 Q4 Arithmetic progressions Solutions - NCERT Maths Class 10

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## Question

Which of the followings are APs? If they form an AP, Find the common difference d and write three more terms.

 \begin{align}\rm{i)}\,\,\,2,4,16.................\end{align} \begin{align}\rm{ii)}\,\,\,2,\frac{5}{2},3,\frac{7}{2}.................\end{align} \begin{align}\rm{iii)}\,\,\, - 1.2, - 3.2, - 5.2, - 7.2.................\end{align} \begin{align}\rm{iv)}{\kern 1pt} \,\,\, - 10, - 6, - 2,2.................\end{align} \begin{align}\rm{v)}\,\,\,3,3 + \sqrt 2 ,\,3 + 2\sqrt 2 ,3 + 3\sqrt 2 .................\end{align} \begin{align}\rm{vi)}\,\,\,0.2,0.22,0.222,0.2222.................\end{align} \begin{align}\rm{vii)}\,\,\,0, - 4, - 8, - 12.................\end{align} \begin{align}\rm{viii)}\,\,\, - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2},.................\end{align} \begin{align}\rm{ix)}\,\,\,1,3,9,27,.................\end{align} \begin{align}\rm{x)}\,\,\,a,2a,3a.4a,.................\end{align} \begin{align}\rm{xi)}\,\,\,a,{a^2},{a^3},{a^4}\end{align} \begin{align}\rm{xii)}\,\,\,\sqrt 2 ,\sqrt 8 ,\sqrt {18} ,\sqrt {132} \end{align} \begin{align}\rm{xiii)}\,\,\,\sqrt 3 ,\sqrt 6 ,\sqrt 9 ,\sqrt {12} \end{align} \begin{align}\rm{xiv)}\,\,\,{1^2},{3^2},{5^2},{7^2}\end{align} \begin{align}\rm{xv)}\,{1^2},{3^2},{5^2},{7^3}\end{align}

Video Solution
Arithmetic Progressions
Ex 5.1 | Question 4

## Text Solution

Reasoning:

General form of an arithmetic progression is \begin{align} a, (a+d), (a+2d), (a+3d)…\end{align} where $$a$$ is the first term and $$d$$ is the common difference.

i) What is Known?

\begin{align} 2, 4, 8, 16…..\end{align}

What is Unknown?

Common difference and next three more terms of AP if it is an AP.

Steps:

The given numbers are $$2,\,4,\,8,\,16$$

First term $$a =2$$

Common difference $$d=a_{2}-a_{1}=4-2=2$$

Common difference $$d=a_{3}-a_{2}=8-4=4$$

\begin{align}({a_{3}-a_{2}) \neq (a_{2}-a_{1})}\end{align}

\begin{align}2, 4, 8, 16\end{align} are not in AP, because the common difference is not equal.

ii) What is Known?

\begin{align}2,\,\frac{5}{2},\,3,\,\frac{7}{2}\end{align}

What is Unknown?

Common difference and next three more terms of AP if it is an AP.

Steps:

The given numbers are \begin{align}2,\,\frac{5}{2},\,3,\,\frac{7}{2}\end{align}

\begin{align}&{\text { First term } a =2} \\&{\text { Common difference } d =a_{2}-a_{1}=\frac{5}{2}-2=\frac{5-4}{2}=\frac{1}{2}} \\&{\text { Common difference } d =a_{3}-a_{2}=3-\frac{5}{2}=\frac{6-5}{2}=\frac{1}{2}}\end{align}

\begin{align}\text{Since }({a_3} - {a_2}) = ({a_2} - {a_1}).\end{align}

\begin{align}2,\,\frac{5}{2},\,3,\,\frac{7}{2}\end{align} forms an AP and common difference is\begin{align}\frac{1}{2}\end{align} .

The next three terms are:

\begin{align} \text { Fifth term } &=a+4 d \\ &=2+4 \times \frac{1}{2} \\ &=2+2 \\ &=4 \end{align}

\begin{align} \text { Sixth term } &=a+5 d \\ &=2+5 \times \frac{1}{2} \\ &=2+\frac{5}{2} \\ &=\frac{4+5}{2} \\ &=\frac{9}{2} \end{align}

\begin{align} \text { Seventh term } &=a+6 d \\ &=2+6 \times \frac{1}{2} \\ &=5 \end{align}

\begin{align}2,\,\frac{5}{2},\,3,\,\frac{7}{2}\end{align} forms an AP and the common difference is \begin{align}\frac{1}{2}\end{align}. The next three terms are \begin{align}4,\,\frac{9}{2}\,,5.\end{align}

iii) What is Known?

\begin{align} - 1.2,\, - 3.2,\, - 5.2,\, - 7.2 \ldots \ldots .\end{align}

What is Unknown?

Whether it forms an AP. If it is find the common difference and the next three terms of AP.

Steps:

The given numbers are \begin{align} - 1.2,\, - 3.2,\, - 5.2,\, - 7.2 \ldots \ldots .\end{align}

\begin{align} \text {First term } a = -1.2\end{align}

\begin{align}\text{Common difference } d ={a_2} - {a_1} = - 3.2 - ( - 1.2) = - 3.2 + 1.2 = - 2\end{align}

\begin{align}\text{Common difference } &={a_3} - {a_2} = - 5.2 - ( - 3.2)\\ &= - 5.2 + 3.2 = - 2 \end{align}

\begin{align} \text{Since }({a_3} - {a_2}) = ({a_2} - {a_1}) . \end{align}

It forms an AP.

\begin{align} \text { The fifth term } &=a+4d \\ &=-1.2+4(-2) \\ &=-1.2-8=-9.2 \end{align}

\begin{align} \text { The sixth term } &=a+5d \\ &=-1.2+5(-2) \\ &=-1.2-10 \\ &=-11.2 \end{align}

\begin{align} \text { The seventh term } &=a+6d \\ &=-1.2+6(-2) \\ &=-1.2-12 \\ &=-13.2 \end{align}

\begin{align} - 1.2,\, - 3.2,\, - 5.2,\, - 7.2\end{align} forms an AP with common difference -2. The next three terms of AP are \begin{align}-9.2, -11.2, -13.2. \end{align}

iv) What is Known?

\begin{align} -10, -6, -2, 2 \end{align}

What is Unknown?

Whether it forms an AP. If it is find the common difference and the next three terms of AP.

Steps:

The given numbers are \begin{align} -10, -6, -2, 2 \end{align}

\begin{align}\text{ First term a =} -10 \end{align}

\begin{align} \text { Common difference } d &=a_{2}-a_{1} \\ &=-6-(-10) \\ &=-6-10 \\ &=4 \end{align}

\begin{align} \text { Common difference } d &=a_{3}-a_{2} \\ &=-2-(-6) \\ &=-2+6 \\ &=4 \end{align}

\begin{align}&{\text { Since } (a_{3}-a_{2})=(a_{2}-a_{1})} \\ &{\text { Fifth Term: } a+4 d=-10+16=6} \\ &{\text { Sixth Term: } a+5 d=-10+20=10} \\& {\text { Seventh Term: } a+6 d=-10+24=14}\end{align}

\begin{align}-10, -6 ,-2, 2 \end{align} forms an AP with common difference $$4$$ and next terms are $$6, 10, 14.$$

v) What is Known?

\begin{align}3,\,\,3 + \sqrt 2 \,\,,3 + 2\sqrt 2 \,\,,3 + 3\sqrt 2 \ldots \ldots \end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align}3,\,\,3 + \sqrt 2 \,\,,3 + 2\sqrt 2 \,\,,3 + 3\sqrt 2 \ldots \ldots \end{align}

\begin{align} \text { Common difference} d &=a_{2}-a_{1} \\ &=3+\sqrt{2}-3 \\ &=\sqrt{2} \end{align}

\begin{align} \text { Common difference } d \mathrm{is} &=a_{3}-a_{2} \\ &=3+2 \sqrt{2}-(3+\sqrt{2}) \\ &=3+2 \sqrt{2}-3-\sqrt{2} \\ &=\sqrt{2} \end{align}

\begin{align} \text {Since }a_{3}-a_{2}=a_{2}-a_{1}\end{align}

So \begin{align}3,\,\,3 + \sqrt 2 \,\,,3 + 2\sqrt 2 \,\,,3 + 3\sqrt 2 \ldots \ldots \end{align} forms an AP with common difference 4.

Next three terms are

\begin{align} \text { Fifth term } &=a+4 d \\ &=3+4 \sqrt{2} \\ \text { Sixth term } &=a+5 d \\ &=3+5 \sqrt{2} \\ \text { Seventh term } &=a+6 d \\ &=3+6 \sqrt{2} \end{align}

It is an AP with common difference \begin{align}\sqrt 2 \end{align} and Next three terms are \begin{align}3 + 4\sqrt 2 ,\,\,3 + 5\sqrt 2 \,\,,3 + 6\sqrt 2 \end{align} ,.

vi) What is Known?

\begin{align}0.2,0.22,0.222,0.2222.....\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers are \begin{align}0.2,0.22,0.222,0.2222.....\end{align}

\begin{align} \text { Common difference } d &=a_{2}-a_{1} \\ &=0.22-0.2 \\ &=0.20 \\ \text { Common difference } d &=a_{3}-a_{2} \\ &=0.222-0.220 \\ &=0.002\\ (a_{3}-a_{2} )&\neq (a_{2}-a_{1})\end{align}

The given list of numbers does not form an AP.

vii) What is Known?

\begin{align} 0,-4,-8,-12……\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align} 0,-4,-8,-12……\end{align}

\begin{align}&\text{Common difference } d = {a_2} - {a_1}= - 4 – 0 = -4\\&\text{Common difference } d = {a_3} - {a_2}= -8 – (-4) = -8 + 4 = -4\end{align}

Since\begin{align}({a_3} - {a_2}) = ({a_2} - {a_1})\end{align} . It forms an AP.

\begin{align} \text { Fifth term } &=a+4 d \\ &=0+4(-4) \\ &=-16 \\ \text { Sixth term } &=a+5 d \\ &=0+5(-4) \\ &=-20 \\ \text { Seventh term } &=a+6 d \\ &=0+6(-4) \\ &=-24 \end{align}

The given numbers form an AP with difference $$-4.$$ The next three terms are \begin{align} -16, -20, -24. \end{align}

viii) What is Known?

\begin{align} - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}.......\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The given list of numbers is \begin{align} - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}.......\end{align}

\begin{align}\text{Common difference } d &= {a_2} - {a_1}\\&= - \frac{1}{2} - \left( { - \frac{1}{2}} \right)\\ &= - \frac{1}{2} + \frac{1}{2}\\&= 0\end{align}

\begin{align}{\text{ Common difference }} d &= {a_3} - {a_2}\\& = - \frac{1}{2} - \left( { - \frac{1}{2}} \right)\\ &= - \frac{1}{2} + \frac{1}{2}\\&= 0\end{align}

\begin{align}\text{Since }({a_3} - {a_2}) = ({a_2} - {a_1})\end{align} .The list of numbers forms an AP.

\begin{align}{\text{The fifth term }} &= a + 4d\\& = - \frac{1}{2} + 4(0)\\ &= - \frac{1}{2}\\{\text{ The sixth term }} &= a + 5d\\ &= - \frac{1}{2} + 5(0)\\ &= - \frac{1}{2}\\{\text{The seventh term}} &= {a} + 6d\\ &= - \frac{1}{2} + 6(0)\\ &= - \frac{1}{2}\end{align}

The given list of numbers form an AP with common difference $$d = 0.$$ Next three terms are \begin{align} - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}, - \frac{1}{2}.\end{align}

ix) What is Known?

\begin{align}1, 3 ,9 ,27.\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers are \begin{align}1, 3 ,9 ,27.\end{align}

\begin{align}\text{Common difference } d = {a_2} - {a_1}= 3 – 1 = 2\end{align}

\begin{align}\text{Common difference } d = {a_3} - {a_2}= 9 -3 =6\end{align}

\begin{align}\text {Since} ({a_2} - {a_1}) \ne ({a_3} - {a_2})\end{align}

The given list of numbers does not form an AP.

x) What is Known?

\begin{align}{a, 2a, 3a, 4a}\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align}{a, 2a, 3a, 4a}.\end{align}

\begin{align} \text { Common difference } d &=a_{2}-a_{1} \\ &=2a-a \\ &=a \\ \text { Common difference } d &=a_{3}-a_{2} \\ &=3a-2a \\ &=a \end{align}

\begin{align}{\text{since }}{a_3} - {a_2} = {a_2} - {a_1}, {a, 2a, 3a, 4a} \text{ forms an AP.}\end{align}

\begin{align}&{\text { The fifth term }=a+4d=a+4a=5a} \\& {\text { The sixth term }=a+5d=a+5a=6a} \\ &{\text { The seventh term }=a+6d=a+6a=7a}\end{align}

The given list of numbers form an AP with common difference $$d =a$$ The next three terms are $$5a, 6a, 7a.$$

xi) What is Known?

\begin{align}a,{a^2},{a^3},{a^4}\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align}a,{a^2},{a^3},{a^4}\end{align}

\begin{align} \text { Common difference } d &=a_{2}-a_{1} \\ &=a^{2}-a \\ &=a(a-1) \end{align}

\begin{align}\text{Common difference } d &= {a_3} - {a_2}\\ &= {a^3} - {a^2}\\ &= {a^2}\left( {a - 1} \right)\end{align}

\begin{align}{\text{Since }}({a_2} - {a_1}) \ne ({a_3} - {a_2})\end{align}

The given list of numbers does not form an AP.

xii) What is Known?

\begin{align}\sqrt 2 ,\sqrt 8 ,\sqrt {18} ,\sqrt {32} \end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align}\sqrt 2 ,\sqrt 8 ,\sqrt {18} ,\sqrt {32} \end{align}

\begin{align}\text{Common difference } d &={a_2} - {a_1}\\&= \sqrt 8 - \sqrt 2 \\ &= \sqrt {4 \times 2} - \sqrt 2 \\ &= 2\sqrt 2 - \sqrt 2 \\ &= \sqrt 2 \end{align}

\begin{align}{\text{Common difference }}d &= {a_3} - {a_2}\\ &= \sqrt {18} - \sqrt 8 \\ &= \sqrt {9 \times 2} - \sqrt {4 \times 2} \\ &= 3\sqrt 2 - 2\sqrt 2 \\& = \sqrt 2 \end{align}

\begin{align}{\text{since }}({a_2} - {a_1}) = ({a_3} - {a_2})\end{align} The given numbers form an AP.

\begin{align}{\text{The fifth term }} &= a + 4d\\ &= \sqrt 2 + 4\sqrt 2 \\ &= 5\sqrt 2 \\ &= \sqrt {25 \times 2} \\ &= \sqrt {50} \\{\text{The sixth term}}& = a + 5d\\ &= \sqrt 2 + 5\sqrt 2 \\ &= 6\sqrt 2 \\ &= \sqrt {36 \times 2} \\ &= \sqrt {72} \\{\text{ The seventh term }} &= {\rm{a}} + 6d\\ &= \sqrt 2 + 6\sqrt 2 \\ &= 7\sqrt 2 \\ &= \sqrt {49 \times 2} \\ &= \sqrt {98} \end{align}

The list of numbers forms an AP with common difference\begin{align}\sqrt 2 \end{align}. Next three terms are \begin{align}\sqrt {50} ,\sqrt {72} ,\sqrt {98} \end{align}

xiii ) What is Known?

\begin{align}\sqrt 3 ,\,\sqrt 6 \,,\sqrt 9 ,\,\sqrt {12} \end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align}\sqrt 3 ,\,\sqrt 6 \,,\sqrt 9 ,\,\sqrt {12} \end{align}

\begin{align}\text{Common difference } d &= {a_2} - {a_1} = \sqrt 6 - \sqrt 3 \\ &= \sqrt {3 \times 2} - \sqrt 3 \\ &= \sqrt 3 \left( {\sqrt 2 - 1} \right)\end{align}

\begin{align}\text{Common difference } d&= {a_3} - {a_2} = \sqrt 9 - \sqrt 6 \\ &= \sqrt {3 \times 3} - \sqrt {3 \times 2} \\ &= \sqrt 3 (\sqrt 3 - \sqrt 2 ){\text{ }}\\{\text{since }}({a_2} - {a_1}) &\ne ({a_3} - {a_2})\end{align}

The given list of numbers does not form an AP.

xiv) What is Known?

\begin{align}{1^2},\,{3^2}\,,{5^2}\,,{7^2}\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers is \begin{align}{1^2},\,{3^2}\,,{5^2}\,,{7^2}\end{align}

\begin{align}\text{Common difference } d= {a_2} - {a_1}= 25 – 1 = 24\end{align}

\begin{align}\text{Common difference } d &={a_3} - {a_2}= 49 – 25 = 24\\ \text{Since }({a_2}- {a_1}) &= ({a_3} - {a_2})\; \text{,they form an AP}\end{align}

\begin{align} \text { The fifth term } &=a+4 d \\ &=1+4 \times 24 \\ &=1+96 \\ &=97 \end{align}

\begin{align} \text { The sixth term } &=a+5 d \\ &=1+5 \times 24 \\ &=1+120 \\ &=121 \end{align}

\begin{align} \text { The seventh term } &=a+6 d \\ &=1+6 \times 24 \\ &=1+144 \\ &=145 \end{align}

The list of numbers form an AP with common difference $$24.$$ The next three terms are $$97, 121,$$ and $$145.$$

xv)What is Known?

\begin{align}{1^2},\,{3^2},\,{5^2},\;{7^3}\end{align}

What is Unknown?

Given list of numbers form an AP or not. If it is find the common difference and the next three terms of AP.

Steps:

The list of numbers \begin{align}{1^2},\,{3^2},\,{5^2},\;{7^3}\end{align}

\begin{align}\text{Common difference } d = {a_2} - {a_1}= 9 – 1 = 8\end{align}

\begin{align}\text{Common difference } d = {a_3} - {a_2}= 25 - 9 = 16\end{align}

\begin{align}\text{Since }({a_2} - {a_1}) \ne ({a_3} - {a_2})\end{align}

The given list of numbers does not form an AP.

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