Rd Sharma XII Vol 1 2019 Solutions for Class 12 Commerce Maths Chapter 13 Derivative As A Rate Measurer are provided here with simple step-by-step explanations. These solutions for Derivative As A Rate Measurer are extremely popular among Class 12 Commerce students for Maths Derivative As A Rate Measurer Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Rd Sharma XII Vol 1 2019 Book of Class 12 Commerce Maths Chapter 13 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Rd Sharma XII Vol 1 2019 Solutions. All Rd Sharma XII Vol 1 2019 Solutions for class Class 12 Commerce Maths are prepared by experts and are 100% accurate.

#### Question 1:

The side of a square sheet is increasing at the rate of 4 cm per minute. At what rate is the area increasing when the side is 8 cm long?

#### Question 2:

An edge of a variable cube is increasing at the rate of 3 cm per second. How fast is the volume of the cube increasing when the edge is 10 cm long?

#### Question 3:

The side of a square is increasing at the rate of 0.2 cm/sec. Find the rate of increase of the perimeter of the square.

#### Question 4:

The radius of a circle is increasing at the rate of 0.7 cm/sec. What is the rate of increase of its circumference?

#### Question 5:

The radius of a spherical soap bubble is increasing at the rate of 0.2 cm/sec. Find the rate of increase of its surface area, when the radius is 7 cm.

#### Question 6:

A balloon which always remains spherical, is being inflated by pumping in 900 cubic centimetres of gas per second. Find the rate at which the radius of the balloon is increasing when the radius is 15 cm.

#### Question 7:

The radius of an air bubble is increasing at the rate of 0.5 cm/sec. At what rate is the volume of the bubble increasing when the radius is 1 cm?

#### Question 8:

A man 2 metres high walks at a uniform speed of 5 km/hr away from a lamp-post 6 metres high. Find the rate at which the length of his shadow increases.

Let AB be the lamp post. Suppose at any time t, the man CD be at a distance of x km from the lamp post and y m be the length of his shadow CE.

#### Question 9:

A stone is dropped into a quiet lake and waves move in circles at a speed of 4 cm/sec. At the instant when the radius of the circular wave is 10 cm, how fast is the enclosed area increasing?

#### Question 10:

A man 160 cm tall, walks away from a source of light situated at the top of a pole 6 m high, at the rate of 1.1 m/sec. How fast is the length of his shadow increasing when he is 1 m away from the pole?

Let AB be the lamp post. Suppose at any time t, the man CD is at a distance of x km from the lamp post and y m is the length of his shadow CE.

#### Question 11:

A man 180 cm tall walks at a rate of 2 m/sec. away, from a source of light that is 9 m above the ground. How fast is the length of his shadow increasing when he is 3 m away from the base of light?

Let AB be the lamp post. Suppose at any time t, the man CD is at a distance x km from the lamp post and y m is the length of his shadow CE.

#### Question 12:

A ladder 13 m long leans against a wall. The foot of the ladder is pulled along the ground away from the wall, at the rate of 1.5 m/sec. How fast is the angle θ between the ladder and the ground is changing when the foot of the ladder is 12 m away from the wall.

Let the bottom of the ladder be at a distance of x m from the wall and its top be at a height of y m from the ground.

Then,

#### Question 13:

A particle moves along the curve y = x2 + 2x. At what point(s) on the curve are the x and y coordinates of the particle changing at the same rate?

#### Question 14:

If y = 7xx3 and x increases at the rate of 4 units per second, how fast is the slope of the curve changing when x = 2?

#### Question 15:

A particle moves along the curve y = x3. Find the points on the curve at which the y-coordinate changes three times more rapidly than the x-coordinate.

#### Question 16:

Find an angle θ
(i) which increases twice as fast as its cosine.
(ii) whose rate of increase twice is twice the rate of decrease of its cosine.

#### Question 17:

The top of a ladder 6 metres long is resting against a vertical wall on a level pavement, when the ladder begins to slide outwards. At the moment when the foot of the ladder is 4 metres from the wall, it is sliding away from the wall at the rate of 0.5 m/sec. How fast is the top-sliding downwards at this instance?
How far is the foot from the wall when it and the top are moving at the same rate?

Let the bottom of the ladder be at a distance of x m from the wall and its top be at a height of y m from the ground.

Here,

#### Question 18:

A balloon in the form of a right circular cone surmounted by a hemisphere, having a diametre equal to the height of the cone, is being inflated. How fast is its volume changing with respect to its total height h, when h = 9 cm.

#### Question 19:

Water is running into an inverted cone at the rate of π cubic metres per minute. The height of the cone is 10 metres, and the radius of its base is 5 m. How fast the water level is rising when the water stands 7.5 m below the base.

#### Question 20:

A man 2 metres high walks at a uniform speed of 6 km/h away from a lamp-post 6 metres high. Find the rate at which the length of his shadow increases.

Let AB be the lamp post. Let at any time t, the man CD be at a distance of x km from the lamp post and y m be the length of his shadow CE.

#### Question 21:

The surface area of a spherical bubble is increasing at the rate of 2 cm2/s. When the radius of the bubble is 6 cm, at what rate is the volume of the bubble increasing?

#### Question 22:

The radius of a cylinder is increasing at the rate 2 cm/sec. and its altitude is decreasing at the rate of 3 cm/sec. Find the rate of change of volume when radius is 3 cm and altitude 5 cm.

#### Question 23:

The volume of metal in a hollow sphere is constant. If the inner radius is increasing at the rate of 1 cm/sec, find the rate of increase of the outer radius when the radii are 4 cm and 8 cm respectively.

#### Question 24:

Sand is being poured onto a conical pile at the constant rate of 50 cm3/ minute such that the height of the cone is always one half of the radius of its base. How fast is the height of the pile increasing when the sand is 5 cm deep.

#### Question 25:

A kite is 120 m high and 130 m of string is out. If the kite is moving away horizontally at the rate of 52 m/sec, find the rate at which the string is being paid out.

#### Question 26:

A particle moves along the curve y = (2/3)x3 + 1. Find the points on the curve at which the y-coordinate is changing twice as fast as the x-coordinate.

#### Question 27:

Find the point on the curve y2 = 8x for which the abscissa and ordinate change at the same rate.

#### Question 28:

The volume of a cube is increasing at the rate of 9 cm3/sec. How fast is the surface area increasing when the length of an edge is 10 cm?

#### Question 29:

The volume of a spherical balloon is increasing at the rate of 25 cm3/sec. Find the rate of change of its surface area at the instant when radius is 5 cm.

#### Question 30:

The length x of a rectangle is decreasing at the rate of 5 cm/minute and the width y is increasing at the rate of 4 cm/minute. When x = 8 cm and y = 6 cm, find the rates of change of (i) the perimeter (ii) the area of the rectangle.

#### Question 31:

A circular disc of radius 3 cm is being heated. Due to expansion, its radius increases at the rate of 0.05 cm/sec. Find the rate at which its area is increasing when radius is 3.2 cm.

#### Question 1:

If a particle moves in a straight line such that the distance travelled in time t is given by s = t3 − 6t2 + 9t + 8. Find the initial velocity of the particle.

#### Question 2:

The volume of a sphere is increasing at 3 cubic centimeter per second. Find the rate of increase of the radius, when the radius is 2 cms.

#### Question 3:

The sides of an equilateral triangle are increasing at the rate of 2 cm/sec. How far is the area increasing when the side is 10 cms?

#### Question 4:

The side of a square is increasing at the rate of 0.1 cm/sec. Find the rate of increase of its perimeter.

#### Question 5:

The radius of a circle is increasing at the rate of 0.5 cm/sec. Find the rate of increase of its circumference.

#### Question 6:

The side of an equilateral triangle is increasing at the rate of $\frac{1}{3}$ cm/sec. Find the rate of increase of its perimeter.

#### Question 7:

Find the surface area of a sphere when its volume is changing at the same rate as its radius.

#### Question 8:

If the rate of change of volume of a sphere is equal to the rate of change of its radius, find the radius of the sphere.

#### Question 9:

The amount of pollution content added in air in a city due to x diesel vehicles is given by P(x) = 0.005x3 + 0.02x2 + 30x. Find the marginal increase in pollution content when 3 diesel vehicles are added and write which value is indicated in the above questions.

Since, marginal increase in the pollution content is the rate of change of total pollution with respect to the number of diesel vehicles, we have

Marginal increase in pollution = $\frac{dP}{dx}=0.015{x}^{2}+0.04x+30$

When x = 3, marginal increase in pollution = $0.015\left(9\right)+0.04\left(3\right)+30=0.135+0.12+30=30.255$

Hence, the required marginal increase in pollution is 30.255 units.

It indicates the pollution level due to x diesel vehicles.

#### Question 10:

A ladder, 5 metre long, standing on a horizontal floor, leans against a vertical wall. If the top of the ladder slides down wards at the rate of 10 cm/sec, then find the rate at which the angle between the floor and ladder is decreasing when lower end of ladder is 2 metres from the wall.

#### Question 1:

If $V=\frac{4}{3}\mathrm{\pi }{r}^{3}$, at what rate in cubic units is V increasing when r = 10 and $\frac{dr}{dt}=0.01$?
(a) π
(b) 4π
(c) 40π
(d) 4π/3

(b) 4π

#### Question 2:

Side of an equilateral triangle expands at the rate of 2 cm/sec. The rate of increase of its area when each side is 10 cm is
(a)
(b)
(c) 10 cm2/sec
(d) 5 cm2/sec

(b)

#### Question 3:

The radius of a sphere is changing at the rate of 0.1 cm/sec. The rate of change of its surface area when the radius is 200 cm is
(a) 8π cm2/sec
(b) 12π cm2/sec
(c) 160π cm2/sec
(d) 200 cm2/sec

(c) 160π cm2/sec

#### Question 4:

A cone whose height is always equal to its diameter is increasing in volume at the rate of 40 cm3/sec. At what rate is the radius increasing when its circular base area is 1 m2?
(a) 1 mm/sec
(b) 0.001 cm/sec
(c) 2 mm/sec
(d) 0.002 cm/sec

(d) 0.002 cm/sec

#### Question 5:

A cylindrical vessel of radius 0.5 m is filled with oil at the rate of 0.25 π m3/minute. The rate at which the surface of the oil is rising, is
(a) 1 m/minute
(b) 2 m/minute
(c) 5 m/minute
(d) 1.25 m/minute

(a) 1 m/minute

#### Question 6:

The distance moved by the particle in time t is given by x = t3 − 12t2 + 6t + 8. At the instant when its acceleration is zero, the velocity is
(a) 42
(b) −42
(c) 48
(d) −48

(b) −42

#### Question 7:

The altitude of a cone is 20 cm and its semi-vertical angle is 30°. If the semi-vertical angle is increasing at the rate of 2° per second, then the radius of the base is increasing at the rate of
(a) 30 cm/sec
(b)
(c) 10 cm/sec
(d) 160 cm/sec

(b)

#### Question 8:

For what values of x is the rate of increase of x3 − 5x2 + 5x + 8 is twice the rate of increase of x?
(a)

(b)

(c)

(d)

(d)

#### Question 9:

The coordinates of the point on the ellipse 16x2 + 9y2 = 400 where the ordinate decreases at the same rate at which the abscissa increases, are
(a) (3, 16/3)
(b) (−3, 16/3)
(c) (3, −16/3)
(d) (3, −3)

(a) (3, 16/3)

#### Question 10:

The radius of the base of a cone is increasing at the rate of 3 cm/minute and the altitude is decreasing at the rate of 4 cm/minute. The rate of change of lateral surface when the radius = 7 cm and altitude 24 cm is
(a) 54π cm2/min
(b) 7π cm2/min
(c) 27 cm2/min
(d) none of these

#### Question 11:

The radius of a sphere is increasing at the rate of 0.2 cm/sec. The rate at which the volume of the sphere increase when radius is 15 cm, is
(a) 12π cm3/sec
(b) 180π cm3/sec
(c) 225π cm3/sec
(d) 3π cm3/sec

(b) 180π cm3/sec

#### Question 12:

The volume of a sphere is increasing at 3 cm3/sec. The rate at which the radius increases when radius is 2 cm, is
(a) $\frac{3}{32\mathrm{\pi }}\mathrm{cm}/\mathrm{sec}$

(b) $\frac{3}{16\mathrm{\pi }}\mathrm{cm}/\mathrm{sec}$

(c) $\frac{3}{48\mathrm{\pi }}\mathrm{cm}/\mathrm{sec}$

(d) $\frac{1}{24\mathrm{\pi }}\mathrm{cm}/\mathrm{sec}$

(b) $\frac{3}{16\mathrm{\pi }}\mathrm{cm}/\mathrm{sec}$

#### Question 13:

The distance moved by a particle travelling in straight line in t seconds is given by s = 45t + 11t2 t3. The time taken by the particle to come to rest is
(a) 9 sec
(b) 5/3 sec
(c) 3/5 sec
(d) 2 sec

(a) 9 sec

#### Question 14:

The volume of a sphere is increasing at the rate of 4π cm3/sec. The rate of increase of the radius when the volume is 288 π cm3, is
(a) 1/4
(b) 1/12
(c) 1/36
(d) 1/9

(c) 1/36

#### Question 15:

If the rate of change of volume of a sphere is equal to the rate of change of its radius, then its radius is equal to

(a) 1 unit

(b)

(c)

(d)

(d)

#### Question 16:

If the rate of change of area of a circle is equal to the rate of change of its diameter, then its radius is equal to
(a)

(b)

(c)

(d) π units

(b)

#### Question 17:

Each side of an equilateral triangle is increasing at the rate of 8 cm/hr. The rate of increase of its area when side is 2 cm, is

(a)

(b)

(c)

(d) none of these

(a)

#### Question 18:

If s = t3 − 4t2 + 5 describes the motion of a particle, then its velocity when the acceleration vanishes, is

(a)

(b)

(c)

(d)

(d)

#### Question 19:

The equation of motion of a particle is s = 2t2 + sin 2t, where s is in metres and t is in seconds. The velocity of the particle when its acceleration is 2 m/sec2, is
(a)

(b)

(c)

(d)

(b)

#### Question 20:

The radius of a circular plate is increasing at the rate of 0.01 cm/sec. The rate of increase of its area when the radius is 12 cm, is
(a) 144 π cm2/sec
(b) 2.4 π cm2/sec
(c) 0.24 π cm2/sec
(d) 0.024 π cm2/sec

(c) 0.24 π cm2/sec

#### Question 21:

The diameter of a circle is increasing at the rate of 1 cm/sec. When its radius is π, the rate of increase of its area is
(a) π cm2/sec
(b) 2π cm2/sec
(c) π2 cm2/sec
(d) 2π2 cm2/sec2

(c) π2 cm2/sec

#### Question 22:

A man 2 metres tall walks away from a lamp post 5 metres height at the rate of 4.8 km/hr. The rate of increase of the length of his shadow is
(a) 1.6 km/hr
(b) 6.3 km/hr
(c) 5 km/hr
(d) 3.2 km/hr

Let AB be the lamp post. Suppose at any time t, the man CD be at a distance of x km from the lamp post and y m be the length of his shadow CE.

#### Question 23:

A man of height 6 ft walks at a uniform speed of 9 ft/sec from a lamp fixed at 15 ft height. The length of his shadow is increasing at the rate of
(a) 15 ft/sec
(b) 9 ft/sec
(c) 6 ft/sec
(d) none of these

(c) 6 ft/sec

Let AB be the lamp post. Suppose at any time t, the man CD be at a distance of x km from the lamp post and y ft be the length of his shadow CE.

#### Question 24:

In a sphere the rate of change of volume is
(a) π times the rate of change of radius
(b) surface area times the rate of change of diameter
(c) surface area times the rate of change of radius
(d) none of these

(c) surface area times the rate of change of radius

#### Question 25:

In a sphere the rate of change of surface area is
(a) 8π times the rate of change of diameter
(b) 2π times the rate of change of diameter
(c) 2π times the rate of change of radius
(d) 8π times the rate of change of radius

(d) 8π times the rate of change of radius

#### Question 26:

A cylindrical tank of radius 10 m is being filled with wheat at the rate of 314 cubic metre per hour. Then the depth of the wheat is increasing at the rate of
(a) 1 m/hr
(b) 0.1 m/hr
(c) 1.1 m/hr
(d) 0.5 m/hr

(a) 1 m/hr

#### Question 1:

Find the rate of change of the total surface area of a cylinder of radius r and height h, when the radius varies.

Let T be the total surface area of a cylinder. Then,

T = $2\mathrm{\pi }r\left(r+h\right)$

Since the radius varies, we differentiate the total surface area w.r.t. radius r.

Now,

$\frac{dT}{dr}=\frac{d}{dr}\left[2\mathrm{\pi }r\left(r+h\right)\right]\phantom{\rule{0ex}{0ex}}⇒\frac{dT}{dr}=\frac{d}{dr}\left(2\mathrm{\pi }{r}^{2}\right)+\frac{d}{dr}\left(2\mathrm{\pi }rh\right)\phantom{\rule{0ex}{0ex}}⇒\frac{dT}{dr}=4\mathrm{\pi }r+2\mathrm{\pi }h\phantom{\rule{0ex}{0ex}}⇒\frac{dT}{dr}=2\mathrm{\pi }\left(r+h\right)$

#### Question 2:

Find the rate of change of the volume of a sphere with respect to its diameter.

Let V and r be the volume and diameter of the sphere, respectively. Then,
V = $\frac{4}{3}\mathrm{\pi }{\left(\mathrm{radius}\right)}^{3}$

$⇒V=\frac{4}{3}\mathrm{\pi }{\left(\frac{r}{2}\right)}^{3}=\frac{1}{6}\mathrm{\pi }{r}^{\mathit{3}}$

$⇒\frac{dV}{dr}=\frac{1}{2}\mathrm{\pi }{r}^{2}\phantom{\rule{0ex}{0ex}}$

#### Question 3:

Find the rate of change of the volume of a sphere with respect to its surface area when the radius is 2 cm.

Let V be the volume of the sphere. Then,

V = $\frac{4}{3}\mathrm{\pi }{r}^{3}$

$⇒\frac{dV}{dr}=4\mathrm{\pi }{r}^{2}$

Let S be the total surface area of sphere. Then,

S = $4\mathrm{\pi }{r}^{2}$

$⇒\frac{dS}{dr}=8\mathrm{\pi }r\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$

#### Question 4:

Find the rate of change of the area of a circular disc with respect to its circumference when the radius is 3 cm.

Let A be the area of the circular disc. Then,

A = $\mathrm{\pi }{r}^{2}$

$⇒\frac{dA}{dr}=2\mathrm{\pi }r\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$
Let C be the circumference of the circular disc. Then,

C = $2\mathrm{\pi }r$

$⇒\frac{dC}{dr}=2\mathrm{\pi }$

#### Question 5:

Find the rate of change of the volume of a cone with respect to the radius of its base.

Let V be the volume of the cone. Then,

V = $\frac{1}{3}\mathrm{\pi }{r}^{2}h$

$⇒\frac{dV}{dr}=\frac{2}{3}\mathrm{\pi }rh\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$

#### Question 6:

Find the rate of change of the area of a circle with respect to its radius r when r = 5 cm.

Let A be area of the circle. Then,
A = $\mathrm{\pi }{r}^{2}$

$⇒\frac{dA}{dr}=2\mathrm{\pi }r\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$
Hence, the rate of change of the area of the circle is $2\mathrm{\pi }r$.
When r = 5 cm,

#### Question 7:

Find the rate of change of the volume of a ball with respect to its radius r. How fast is the volume changing with respect to the radius when the radius is 2 cm?

Let V be the volume of the spherical ball. Then,
V = $\frac{4}{3}\mathrm{\pi }{r}^{3}$

$⇒\frac{dV}{dr}=4\mathrm{\pi }{r}^{2}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$
Thus, the rate of change of the volume of the sphere is $4\mathrm{\pi }{r}^{2}$.

#### Question 8:

The total cost C (x) associated with the production of x units of an item is given by C (x) = 0.007x3 − 0.003x2 + 15x + 4000. Find the marginal cost when 17 units are produced.

Since the marginal cost is the rate of change of total cost with respect to its output,

Marginal Cost (MC) = $\frac{dC}{dx}\left(x\right)=\frac{d}{dx}\left(0.007{x}^{3}-0.003{x}^{2}+15x+4000\right)=0.021{x}^{2}-0.006x+15$
When x = 17,
Marginal Cost (MC) =

#### Question 9:

The total revenue received from the sale of x units of a product is given by R (x) = 13x2 + 26x + 15. Find the marginal revenue when x = 7.

Since the marginal revenue is the rate of change of total revenue with respect to its output,
Marginal Revenue (MR) = $\frac{dR}{dx}\left(x\right)=\frac{d}{dx}\left(13{x}^{2}+26x+15\right)=26x+26$

When x = 7,
Marginal Revenue (MR)

#### Question 10:

The money to be spent for the welfare of the employees of a firm is proportional to the rate of change of its total revenue (Marginal revenue). If the total revenue (in rupees) recieved from the sale of x units of a product is given by R(x) = 3x2 + 36x + 5, find the marginal revenue, when x = 5, and write which value does the question indicate.