NCERT Solutions
Class 12 - Mathematics Part-1 - Chapter 5: CONTINUITY AND DIFFERENTIABILITY
Exercise 5.1

Question. 1

Prove that the function \(f(x)=5x-3\) is continuous at \(x=0\), at \(x=-3\) and at \(x=5\).

Answer:

Question. 2

Examine the continuity of the function \(f(x)=2x^2-1\) at \(x=3\).

Answer:

\(f\) is continuous at \(x=3\).

Question. 3

Examine the following functions for continuity:

(a) \(f(x)=x-5\)

(b) \(f(x)=\frac{1}{x-5},\; x\ne 5\)

(c) \(f(x)=\frac{x^2-25}{x+5},\; x\ne -5\)

(d) \(f(x)=|x-5|\)

Answer:

(a), (b), (c) and (d) are all continuous functions.

Question. 4

Prove that the function \(f(x)=x^n\) is continuous at \(x=n\), where \(n\) is a positive integer.

Answer:

Question. 5

Is the function \(f\) defined by

\[ f(x)=\begin{cases} x, & x\le 1 \\ 5, & x>1 \end{cases} \]

continuous at \(x=0\)? At \(x=1\)? At \(x=2\)?

Answer:

\(f\) is continuous at \(x=0\) and \(x=2\); not continuous at \(x=1\).

Question. 6

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} 2x+3, & x\le 2 \\ 2x-3, & x>2 \end{cases} \]

Answer:

Discontinuous at \(x=2\).

Question. 7

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} |x|+3, & x\le -3 \\ -2x, & -3

Answer:

Discontinuous at \(x=3\).

Question. 8

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} \frac{|x|}{x}, & x\ne 0 \\ 0, & x=0 \end{cases} \]

Answer:

Discontinuous at \(x=0\).

Question. 9

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} \frac{x}{|x|}, & x<0 \\ -1, & x\ge 0 \end{cases} \]

Answer:

No point of discontinuity.

Question. 10

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} x+1, & x\ge 1 \\ x^2+1, & x<1 \end{cases} \]

Answer:

No point of discontinuity.

Question. 11

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} x^3-3, & x\le 2 \\ x^2+1, & x>2 \end{cases} \]

Answer:

No point of discontinuity.

Question. 12

Find all points of discontinuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} x^{10}-1, & x\le 1 \\ x^2, & x>1 \end{cases} \]

Answer:

\(f\) is discontinuous at \(x=1\).

Question. 13

Is the function defined by

\[ f(x)=\begin{cases} x+5, & x\le 1 \\ x-5, & x>1 \end{cases} \]

a continuous function?

Answer:

\(f\) is not continuous at \(x=1\).

Question. 14

Discuss the continuity of the function \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} 3, & 0\le x\le 1 \\ 4, & 1

Answer:

\(f\) is not continuous at \(x=1\) and \(x=3\).

Question. 15

Discuss the continuity of the function \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} 2x, & x<0 \\ 0, & 0\le x\le 1 \\ 4x, & x>1 \end{cases} \]

Answer:

\(x=1\) is the only point of discontinuity.

Question. 16

Discuss the continuity of the function \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} -2, & x\le -1 \\ 2x, & -11 \end{cases} \]

Answer:

Continuous

Question. 17

Find the relationship between \(a\) and \(b\) so that the function \(f\) defined by

\[ f(x)=\begin{cases} ax+1, & x\le 3 \\ bx+3, & x>3 \end{cases} \]

is continuous at \(x=3\).

Answer:

\(a=b+\frac{2}{3}\)

Question. 18

For what value of \(\lambda\) is the function defined by

\[ f(x)=\begin{cases} \lambda(x^2-2x), & x\le 0 \\ 4x+1, & x>0 \end{cases} \]

continuous at \(x=0\)? What about continuity at \(x=1\)?

Answer:

For no value of \(\lambda\), \(f\) is continuous at \(x=0\) but \(f\) is continuous at \(x=1\) for any value of \(\lambda\).

Question. 19

Show that the function defined by \(g(x)=x-[x]\) is discontinuous at all integral points. Here \([x]\) denotes the greatest integer less than or equal to \(x\).

Answer:

Question. 20

Is the function defined by \(f(x)=x^2-\sin x+5\) continuous at \(x=\pi\)?

Answer:

\(f\) is continuous at \(x=\pi\).

Question. 21

Discuss the continuity of the following functions:

(a) \(f(x)=\sin x+\cos x\)

(b) \(f(x)=\sin x-\cos x\)

(c) \(f(x)=\sin x\cdot\cos x\)

Answer:

(a), (b) and (c) are all continuous

Question. 22

Discuss the continuity of the cosine, cosecant, secant and cotangent functions.

Answer:

Cosine function is continuous for all \(x\in\mathbb{R}\); cosecant is continuous except for \(x=n\pi,\; n\in\mathbb{Z}\); secant is continuous except for \(x=\frac{(2n+1)\pi}{2},\; n\in\mathbb{Z}\) and cotangent function is continuous except for \(x=n\pi,\; n\in\mathbb{Z}\).

Question. 23

Find all points of discontinuity of \(f\), where

\[ f(x)=\begin{cases} \frac{\sin x}{x}, & x<0 \\ x+1, & x\ge 0 \end{cases} \]

Answer:

There is no point of discontinuity.

Question. 24

Determine if \(f\) defined by

\[ f(x)=\begin{cases} x^2\sin\frac{1}{x}, & x\ne 0 \\ 0, & x=0 \end{cases} \]

is a continuous function.

Answer:

Question. 25

Examine the continuity of \(f\), where \(f\) is defined by

\[ f(x)=\begin{cases} \sin x-\cos x, & x\ne 0 \\ -1, & x=0 \end{cases} \]

Answer:

\(f\) is continuous for all \(x\in\mathbb{R}\).

Question. 26

Find the value of \(k\) so that the function \(f\) is continuous at \(x=\frac{\pi}{2}\), where

\[ f(x)=\begin{cases} \dfrac{k\cos x}{\pi-2x}, & x\ne \dfrac{\pi}{2} \\ 3, & x=\dfrac{\pi}{2} \end{cases} \]

Answer:

\(k=6\)

Question. 27

Find the value of \(k\) so that the function \(f\) is continuous at \(x=2\), where

\[ f(x)=\begin{cases} kx^2, & x\le 2 \\ 3, & x>2 \end{cases} \]

Answer:

\(k=\frac{3}{4}\)

Question. 28

Find the value of \(k\) so that the function \(f\) is continuous at \(x=\pi\), where

\[ f(x)=\begin{cases} kx+1, & x\le \pi \\ \cos x, & x>\pi \end{cases} \]

Answer:

\(k=-\frac{2}{\pi}\)

Question. 29

Find the value of \(k\) so that the function \(f\) is continuous at \(x=5\), where

\[ f(x)=\begin{cases} kx+1, & x\le 5 \\ 3x-5, & x>5 \end{cases} \]

Answer:

\(k=\frac{9}{5}\)

Question. 30

Find the values of \(a\) and \(b\) such that the function defined by

\[ f(x)=\begin{cases} 5, & x\le 2 \\ ax+b, & 2

is a continuous function.

Answer:

\(a=2,\; b=1\)

Question. 31

Show that the function defined by \(f(x)=\cos(x^2)\) is a continuous function.

Answer:

Question. 32

Show that the function defined by \(f(x)=|\cos x|\) is a continuous function.

Answer:

Question. 33

Examine that \(\sin|x|\) is a continuous function.

Answer:

Question. 34

Find all the points of discontinuity of \(f\) defined by \(f(x)=|x|-|x+1|\).

Answer:

There is no point of discontinuity.

NCERT Solutions Class 12 – Mathematics Part-1 – Chapter 5: CONTINUITY AND DIFFERENTIABILITY – Exercise 5.1 | Detailed Answers