Question:

The volume V and depth x of water in a vessel are connected by the relation $V=5x-\frac{x^{2}}{6} $ and the volume of water is increasing, at the rate of $5 cm^{3}/sec,$ when x = 2 cm. The rate at which the depth of water is increasing, is

Updated On: Apr 19, 2024
  • $\frac{5}{18} cm/sec$
  • $\frac{1}{4} cm/sec$
  • $\frac{5}{16} cm/sec$
  • None of these
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The Correct Option is D

Solution and Explanation

$V=5x-\frac{x^{2}}{6} \Rightarrow \frac{d v}{d t}=5\frac{d x}{d t}\frac{x}{3}. \frac{d x}{d t}$
$\Rightarrow \frac{d x}{d t}=\frac{\frac{d v}{d t}}{\left(5-\frac{x}{3}\right)}$
$\Rightarrow\, \left(\frac{d x}{d t}\right)_{x=2} =\frac{5}{5-\frac{2}{3}}=\frac{15}{13} cm/sec .$
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Concepts Used:

Application of Derivatives

Various Applications of Derivatives-

Rate of Change of Quantities:

If some other quantity ‘y’ causes some change in a quantity of surely ‘x’, in view of the fact that an equation of the form y = f(x) gets consistently pleased, i.e, ‘y’ is a function of ‘x’ then the rate of change of ‘y’ related to ‘x’ is to be given by 

\(\frac{\triangle y}{\triangle x}=\frac{y_2-y_1}{x_2-x_1}\)

This is also known to be as the Average Rate of Change.

Increasing and Decreasing Function:

Consider y = f(x) be a differentiable function (whose derivative exists at all points in the domain) in an interval x = (a,b).

  • If for any two points x1 and x2 in the interval x such a manner that x1 < x2, there holds an inequality f(x1) ≤ f(x2); then the function f(x) is known as increasing in this interval.
  • Likewise, if for any two points x1 and x2 in the interval x such a manner that x1 < x2, there holds an inequality f(x1) ≥ f(x2); then the function f(x) is known as decreasing in this interval.
  • The functions are commonly known as strictly increasing or decreasing functions, given the inequalities are strict: f(x1) < f(x2) for strictly increasing and f(x1) > f(x2) for strictly decreasing.

Read More: Application of Derivatives