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A block of ice in the shape of a cube melts uniformly maintaining its shape. The volume of a cube given a side length is given by the formula V = S^3. At the moment S = 2 inches, the volume of the cube is decreasing at a rate of 5 cubic inches per minute. What is the rate of change of the side length of the cube with respect to time, in inches per minute, at the moment when S = 2 inches?

Sagot :

Answer:

Ds/dt  = 1,25 in/minute

Step-by-step explanation:

s   is side of the cube

V(c) = s³

Differentiation on both sides of the equation with respect to time.

DV(c) / dt  = 2*s* Ds/dt          (1)

In that equation:

s  = 2

DV(c) /dt  = 5

By subtitution in equation (1)

5  = 2*s*Ds/dt

2*2*Ds/dt   = 5  

Ds/dt   = 5/4      Ds/dt  = 1,25 in/minute

The rate of change of the side length of the cube with respect to time, in inches per minute, at the moment when S = 2 inches is [tex]\dfrac{5}{12} \: \rm in^3/min[/tex]

How to calculate the instantaneous rate of growth of a function?

Suppose that a function is defined as;

[tex]y = f(x)[/tex]

Then, suppose that we want to know the instantaneous rate of the growth of the function with respect to the change in x, then its instantaneous rate is given as:

[tex]\dfrac{dy}{dx} = \dfrac{d(f(x))}{dx}[/tex]

Let the rate of change of volume V with respect to time t is given by:

[tex]\dfrac{dV}{dt}[/tex]

And let the rate of change of volume S with respect to time t is given by:

[tex]\dfrac{dS}{dt}[/tex]

The relation between V and S is [tex]V = S^3[/tex]. Using this value, and the chain rule of differentiation, we get:

[tex]\dfrac{dV}{dt} = \dfrac{dS^3}{dt} = 3S^2\dfrac{dS}{dt} = 3S^2\dfrac{dS}{dt}\\\\\dfrac{dS}{dt} = \dfrac{1}{3S^2} \dfrac{dV}{dt}[/tex]

At S = 2, we are given that:  [tex]\dfrac{dV}{dt} = 5 \: \rm in^3/min[/tex]

Putting these values in the equation for rate of S, we get:

[tex]\dfrac{dS}{dt} = \dfrac{1}{3S^2} \dfrac{dV}{dt}\\\\\dfrac{dS}{dt} = \dfrac{1}{3(2)^2}\times 5 = \dfrac{5}{12} \: \rm in^3/min[/tex]

Thus, the rate of change of the side length of the cube with respect to time, in inches per minute, at the moment when S = 2 inches is [tex]\dfrac{5}{12} \: \rm in^3/min[/tex]

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