Question:

If $y=4x-5$ is tangent to the curve $y^{2}=px^{3}+q$ at $\left(\right.2, \, 3\left.\right)$ then $\left(\right.p,q\left.\right)$ is

Updated On: Jul 28, 2022
  • $\left(\right.2, \, 7\left.\right)$
  • $\left(\right.-2, \, 7\left.\right)$
  • $\left(\right.-2, \, -7\left.\right)$
  • $\left(\right.2, \, -7\left.\right)$
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The Correct Option is D

Solution and Explanation

Curve is $y^{2}=px^{3}+q$ $\therefore \, \, 2y\frac{d y}{d x}=3px^{2}$ $\Rightarrow \, \, \left(\frac{d y}{d x}\right)_{\left(\right. 2,3 \left.\right)}=\frac{3 p . 4}{2.3}$ $\Rightarrow \, \, \, 4=2p$ $\Rightarrow \, \, \, p=2$ Also, curve is passing through $\left(\right.2, \, 3\left.\right)$ $\therefore \, \, 9=8p+q$ $\Rightarrow \, \, \, q=-7$ $? \, \, \left(p , \, q\right)$ is $\left(2 , \, - 7\right)$
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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