# Class 12 RD Sharma Solutions – Chapter 14 Differentials, Errors and Approximations – Exercise 14.1 | Set 2

### Question 9: Using differentials, find the approximate values of the following:

### (xiv)

**Solution:**

Considering the function as

y = f(x) = cos x

Taking x = π/3, and

x+△x = 11π/36

△x = 11π/36-π/3 = -π/36

y = cos x

= cos (π/3) = 0.5

= – sin x

= – sin (π/3) = -0.86603

△y = dy = dx

△y = (-0.86603) (-π/36)

△y = 0.0756

Hence, = 0.5+0.0756 = 0.5755

### (xv)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 81, and

x+△x = 80

△x = 80-81 = -1

△y = dy = dx

△y = △x

△y = (-1) = = -0.009259

Hence,

= y+△y = 3 + (-0.009259) = 2.99074

### (xvi)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 27, and

x+△x = 29

△x = 29-27 = 2

△y = dy = dx

△y = △x

△y = (2) = 0.074

Hence,

= y+△y = 3+0.074 = 3.074

### (xvii)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 64, and

x+△x = 66

△x = 66-64 = 2

△y = dy = dx

△y = △x

△y = (2) = 0.042

Hence,

= y+△y = 4+0.042 = 4.042

### (xviii)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 25, and

x+△x = 26

△x = 26-25 = 1

△y = dy = dx

△y = △x

△y = (1) = 0.1

Hence,

= y+△y = 5 + 0.1 = 5.1

### (xix)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 36, and

x+△x = 37

△x = 37-36 = 1

△y = dy = dx

△y = △x

△y = (1) = 0.0833

Hence,

= y+△y = 6 + 0.0833 = 6.0833

### (xx)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 0.49, and

x+△x = 0.48

△x = 0.48-0.49 = -0.01

△y = dy = dx

△y = △x

△y = (-0.01) = -0.007143

Hence,

= y+△y = 0.7 + (-0.007143) = 0.693

### (xxi)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 81, and

x+△x = 82

△x = 82-81 = 1

△y = dy = dx

△y = △x

△y = = 0.009259

Hence,

= y+△y = 3 + 0.009259 = 3.009259

### (xxii)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 16/81, and

x+△x = 17/81

△x = 17/81-16/81 = 1/81

△y = dy = dx

△y = △x

△y = = 0.01042

Hence,

= y+△y = 2/3 + 0.01042 = 0.6771

### (xxiii)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 32, and

x+△x = 33

△x = 33-32 = 1

△y = dy = dx

△y = △x

△y = = 0.0125

Hence,

= y+△y = 2 + 0.0125 = 2.0125

### (xxiv)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 36, and

x+△x = 36.6

△x = 36.6-36 = 0.6

△y = dy = dx

△y = △x

△y = (0.6) = 0.05

Hence,

= y+△y = 6 + 0.05 = 6.05

### (xxv)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 27, and

x+△x = 25

△x = 25-27 = -2

△y = dy = dx

△y = △x

△y = (-2) = -0.07407

Hence,

= y+△y = 3+(-0.07407) = 2.9259

### (xxvi)

**Solution:**

Considering the function as

y = f(x) =

Taking x = 49, and

x+△x = 49.5

△x = 49.5-49 = 0.5

△y = dy = dx

△y = △x

△y = (0.5) = 0.0357

Hence,

= y+△y = 7 + 0.0357 = 7.0357

### Question 10: Find the appropriate value of f(2.01), where f(x) = 4x^{2}+5x+2

**Solution:**

Considering the function as

y = f(x) = 4x

^{2}+5x+2Taking x = 2, and

x+△x = 2.01

△x = 2.01-2 = 0.01

y = 4x

^{2}+5x+2= 4(2)

^{2}+5(2)+2 = 28= 8x+5

= 8(2)+5 = 21

△y = dy = dx

△y = (21) △x

△y = (21) (0.01) = 0.21

Hence,

f(2.01) = y+△y = 28 + 0.21 = 28.21

### Question 11: Find the appropriate value of f(5.001), where f(x) = x^{3}-7x^{2}+15

**Solution:**

Considering the function as

y = f(x) = x

^{3}-7x^{2}+15Taking x = 5, and

x+△x = 5.001

△x =5.001-5 = 0.001

y = x

^{3}-7x^{2}+15= (5)

^{3}-7(5)^{2}+15 = -35= 3x

^{2}-14x= 3(5)

^{2}-14(5) = 5△y = dy = dx

△y = (5) △x

△y = (5) (0.001) = 0.005

Hence,

f(5.001) = y+△y = -35 + 0.005 = -34.995

### Question 12: Find the appropriate value of log_{10} 1005, given that log_{10} e=0.4343

**Solution:**

Considering the function as

y = f(x) = log

_{10}xTaking x = 1000, and

x+△x = 1005

△x =1005-1000 = 5

y = log

_{10}x == log

_{10}1000 = 3= 0.0004343

△y = dy = dx

△y = (0.0004343) △x

△y = (0.0004343) (5) = 0.0021715

Hence, log

_{10}1005 = y+△y = 3 + 0.0021715 = 3.0021715

### Question 13: If the radius of a sphere is measured as 9cm with an error of 0.03m, find the approximate error in calculating its surface area.

**Solution:**

According to the given condition,

As, Surface area = 4πx

^{2}Let △x be the change in the radius and △y be the change in the surface area

x = 9

△x = 0.03m = 3cm

x+△x = 9+3 = 12cm

= 4πx

^{2}= 4π(9)^{2}= 324 π= 8πx

= 8π(9) = 72π

△y = dy = dx

△y = (72π) △x

△y = (72π) (3) = 216 π

Hence, approximate error in surface area of the sphere is 216 π cm^{2}

### Question 14: Find the approximate change in the surface area of a cube as side x meters caused by decreasing the side by 1%.

**Solution:**

According to the given condition,

As, Surface area = 6x

^{2}Let △x be the change in the length and △y be the change in the surface area

△x/x × 100 = 1

= 6(2x) = 12x

△y = △x

△y = (12x) (x/100)

△y = 0.12 x

^{2}Hence, the approximate change in the surface area of a cubical box is 0.12 x

^{2}m^{2}

### Question 15: If the radius of a sphere is measured as 7m with an error of 0.02m, find the approximate error in calculating its volume.

**Solution:**

According to the given condition,

As, Volume of sphere = πx

^{3}Let △x be the error in the radius and △y be the error in the volume

x = 7

△x = 0.02 cm

π(3x

^{2}) = 4πx^{2}= 4π(7)

^{2}= 196π△y = dy = dx

△y = (196π) △x

△y = (196π) (0.02) = 3.92 π

Hence, approximate error in volume of the sphere is 3.92 π cm

^{2}

### Question 16: Find the approximate change in the volume of a cube as side x meters caused by increasing the side by 1%.

**Solution:**

According to the given condition,

As, Volume of cube = x

^{3}Let △x be the change in the length and △y be the change in the volume

△x/x × 100 = 1

= 3x

^{2}△y = △x

△y = (3x

^{2}) (x/100)△y = 0.03 x

^{3}

Hence, the approximate change in the volume of a cubical box is 0.03 x^{3}m^{3}