# Ex.5.1 Q3 Arithmetic progressions Solutions - NCERT Maths Class 10

## Question

For the following APs, write the first term and the common difference:

\(\begin{align} &\rm{i)}\,\,3,1, - 1, - 3 \ldots \ldots \\ &\rm{ii)}\,\, - 5, - 1,3,7 \ldots \ldots \\ &\rm{iii)}\,\,\frac{1}{3},\frac{5}{3},\frac{9}{3},\frac{{13}}{3} \ldots \ldots.\quad \quad \\ &\rm{iv)}\,\,0.6,1.7,2.8,3.9 \ldots \end{align}\)

## 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 difference.

**i) What is known?**

\(3,1,-1,-3\) are in AP.

**What is Unknown?**

First term and the common difference of the AP.

**Steps:**

The AP is \(\begin{align} 3, 1 ,-1, -3.\end{align}\)

First term \(\begin{align}{a = 3}.\end{align}\)

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

First term is \(3\) and the common difference is \(-2.\)

**ii) What is known?**

\(\begin{align}-5, -1, 3, 7\end{align}\) are in AP

**What is Unknown?**

First term and the common difference of the AP.

**Step:**

The AP is \(\begin{align}-5, -1, 3, 7.\end{align}\)

First term\(\begin{align} a = - 5\end{align}\)

\[\begin{align} \text{Common difference} & = {a_2} - {a_1}\\&= -1 – (-5)\\&= -1 + 5\\& = 4\end{align}\]

First term \(-5\) and the common difference is \(4.\)

**iii) What is known?**

\(\begin{align}\frac{1}{3},\frac{5}{3},\frac{9}{3},\frac{{13}}{3}\end{align}\) are in AP.

**What is Unknown?**

First term and the common difference of the AP.

**Steps:**

The AP is \(\begin{align}\frac{1}{3},\frac{5}{3},\frac{9}{3},\frac{{13}}{3}\end{align}\)

First term \(\begin{align} a = \frac{1}{3}\end{align}\)

\[\begin{align}\text{Common difference} &= {a_2} - {a_1}\\&= \frac{5}{3} - \frac{1}{3}\\&= \frac{{5 - 1}}{3}\\&= \frac{4}{3}\end{align}\]

First term is \(\begin{align}\frac{1}{3}\end{align}\) and the common difference is \(\begin{align}\frac{4}{3}\end{align}\)

**iv) What is known?**

\(\begin{align}{\rm{0}}{\rm{.6,}}\,\,{\rm{1}}{\rm{.7,}}\,\,{\rm{2}}{\rm{.8,}}\,\,{\rm{3}} {\rm{.9}}\end{align}\) are in AP.

**What is Unknown?**

First term and the common difference of the AP.

**Steps:**

The AP is \(\begin{align} {\rm{0}}{\rm{.6,}}\,\,{\rm{1}}{\rm{.7,}}\,\,{\rm{2}}{\rm{.8,}}\,\,{\rm{3}}{\rm{.9}}\end{align}\)

First term \(\begin{align} a = 0.6 \end{align}\)

\[\begin{align} \text{ Common difference}&= {a_2} - {a_1}\\&= 1.7 – 0.6\\&= 1.1\end{align}\]

First term \(= 0.6\) and the common difference \(= 1.1\)