NCERT Class 12 Part 1 - Math

Pawan K. Jain

Chapter 6

Application of Derivatives - all with Video Answers

Educators

Section 1

Introduction

Problem 1

Find the rate of change of the area of a circle with respect to its radius $r$ when
(a) $r=3 \mathrm{~cm}$
(b) $r=4 \mathrm{~cm}$

Tanishq G.

Tanishq G.

Numerade Educator

Problem 2

The volume of a cube is increasing at the rate of $8 \mathrm{~cm}^{3} / \mathrm{s}$. How fast is the surface area increasing when the length of an edge is $12 \mathrm{~cm}$ ?

Tanishq G.

Numerade Educator

Problem 3

The radius of a circle is increasing uniformly at the rate of $3 \mathrm{~cm} / \mathrm{s}$. Find the rate at which the area of the circle is increasing when the radius is $10 \mathrm{~cm}$.

Tanishq G.

Numerade Educator

Problem 4

An edge of a variable cube is increasing at the rate of $3 \mathrm{~cm} / \mathrm{s}$. How fast is the volume of the cube increasing when the edge is $10 \mathrm{~cm}$ long?

Tanishq G.

Numerade Educator

Problem 5

A stone is dropped into a quiet lake and waves move in circles at the speed of $5 \mathrm{~cm} / \mathrm{s}$. At the instant when the radius of the circular wave is $8 \mathrm{~cm}$, how fast is the enclosed area increasing?

Tanishq G.

Numerade Educator

Problem 6

The radius of a circle is increasing at the rate of $0.7 \mathrm{~cm} / \mathrm{s}$. What is the rate of increase of its circumference?

Tanishq G.

Numerade Educator

Problem 7

The length $x$ of a rectangle is decreasing at the rate of $5 \mathrm{~cm} /$ minute and the width $y$ is increasing at the rate of $4 \mathrm{~cm} /$ minute. When $x=8 \mathrm{~cm}$ and $y=6 \mathrm{~cm}$, find the rates of change of (a) the perimeter, and (b) the area of the rectangle.

Tanishq G.

Numerade Educator

Problem 8

A balloon, which always remains spherical on inflation, is being inflated by pumping in 900 cubic centimetres of gas per second. Find the rate at which the radius of the balloon increases when the radius is $15 \mathrm{~cm}$.

Tanishq G.

Numerade Educator

Problem 9

A balloon, which always remains spherical has a variable radius. Find the rate at which its volume is increasing with the radius when the later is $10 \mathrm{~cm}$.

Tanishq G.

Numerade Educator

Problem 10

A ladder $5 \mathrm{~m}$ long is leaning against a wall. The bottom of the ladder is pulled along the ground, away from the wall, at the rate of $2 \mathrm{~cm} / \mathrm{s}$. How fast is its height on the wall decreasing when the foot of the ladder is $4 \mathrm{~m}$ away from the wall?

Tanishq G.

Numerade Educator

Problem 11

A particle moves along the curve $6 y=x^{3}+2$. Find the points on the curve at which the $y$ -coordinate is changing 8 times as fast as the $x$ -coordinate.

Tanishq G.

Numerade Educator

Problem 12

The radius of an air bubble is increasing at the rate of $\frac{1}{2} \mathrm{~cm} / \mathrm{s}$. At what rate is the volume of the bubble increasing when the radius is $1 \mathrm{~cm}$ ?

Tanishq G.

Numerade Educator

Problem 13

A balloon, which always remains spherical, has a variable diameter $\frac{3}{2}(2 x+1)$. Find the rate of change of its volume with respect to $x$.

Tanishq G.

Numerade Educator

Problem 14

Sand is pouring from a pipe at the rate of $12 \mathrm{~cm}^{3} / \mathrm{s}$. The falling sand forms a cone on the ground in such a way that the height of the cone is always one-sixth of the radius of the base. How fast is the height of the sand cone increasing when the height is $4 \mathrm{~cm}$ ?

Tanishq G.

Numerade Educator

Problem 15

The total cost $\mathrm{C}(x)$ in Rupees associated with the production of $x$ units of an item is given by
$$C(x)=0.007 x^{3}-0.003 x^{2}+15 x+4000.$$
Find the marginal cost when 17 units are produced.

Tanishq G.

Numerade Educator

Problem 16

The total revenue in Rupees received from the sale of $x$ units of a product is given by
$$\mathrm{R}(x)=13 x^{2}+26 x+15.$$
Find the marginal revenue when $x=7$.

Tanishq G.

Numerade Educator

Problem 17

The rate of change of the area of a circle with respect to its radius $r$ at $r=6 \mathrm{~cm}$ is
(A) $10 \pi$
(B) $12 \pi$
(C) $8 \pi$
(D) $11 \pi$

Tanishq G.

Numerade Educator

Problem 18

The total revenue in Rupees received from the sale of $x$ units of a product is given by
$\mathrm{R}(x)=3 x^{2}+36 x+5$. The marginal revenue, when $x=15$ is
(A) 116
(B) 96
(C) 90
(D) 126

Tanishq G.

Numerade Educator