Question

If the sub-normal at any point on $$y = {a^{1 - n}}{x^n}$$   is of constant length, then the value of $$n$$ is :

A. $$\frac{1}{4}$$
B. $$1$$
C. $$\frac{1}{3}$$
D. $$\frac{1}{2}$$  
Answer :   $$\frac{1}{2}$$
Solution :
$$\eqalign{ & y = {a^{1 - n}}{x^n}{\text{ or }}\frac{{dy}}{{dx}} = {a^{1 - n}}n{x^{n - 1}} \cr & {\text{Sub - normal}} = \left| {y\frac{{dy}}{{dx}}} \right| \cr & = \left| {y{a^{1 - n}}n{x^{n - 1}}} \right| \cr & = \left| {{a^{1 - n}}{x^n}{a^{1 - n}}n{x^{n - 1}}} \right| \cr & = \left| {{a^{2 - 2n}}{x^{2n - 1}}} \right| \cr & {\text{which is constant if }}2n - 1 = 0{\text{ or }}n = \frac{1}{2} \cr} $$

Releted MCQ Question on
Calculus >> Application of Derivatives

Releted Question 1

If  $$a + b + c = 0,$$    then the quadratic equation $$3a{x^2}+ 2bx + c = 0$$     has

A. at least one root in $$\left[ {0, 1} \right]$$
B. one root in $$\left[ {2, 3} \right]$$  and the other in $$\left[ { - 2, - 1} \right]$$
C. imaginary roots
D. none of these
Releted Question 2

$$AB$$  is a diameter of a circle and $$C$$ is any point on the circumference of the circle. Then

A. the area of $$\Delta ABC$$  is maximum when it is isosceles
B. the area of $$\Delta ABC$$  is minimum when it is isosceles
C. the perimeter of $$\Delta ABC$$  is minimum when it is isosceles
D. none of these
Releted Question 3

The normal to the curve $$x = a\left( {\cos \theta + \theta \sin \theta } \right),y = a\left( {\sin \theta - \theta \cos \theta } \right)$$        at any point $$'\theta '$$ is such that

A. it makes a constant angle with the $$x - $$axis
B. it passes through the origin
C. it is at a constant distance from the origin
D. none of these
Releted Question 4

If $$y = a\ln x + b{x^2} + x$$     has its extremum values at $$x = - 1$$  and $$x = 2,$$  then

A. $$a = 2,b = - 1$$
B. $$a = 2,b = - \frac{1}{2}$$
C. $$a = - 2,b = \frac{1}{2}$$
D. none of these

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