Question

The straight line $$\frac{x}{a} + \frac{y}{b} = 2$$   touches the curve $${\left( {\frac{x}{a}} \right)^n} + {\left( {\frac{y}{b}} \right)^n} = 2$$    at the point $$\left( {a,\,b} \right)$$  for :

A. $$n = 1,\,2$$
B. $$n = 3,\,4,\, - 5$$
C. $$n = 1,\,2,\,3$$
D. any value of $$n$$  
Answer :   any value of $$n$$
Solution :
The point $$\left( {a,\,b} \right)$$  lies on both the straight line and the given curve $${\left( {\frac{x}{a}} \right)^n} + {\left( {\frac{y}{b}} \right)^n} = 2$$
Differentiating the equation, we get
$$\eqalign{ & \frac{{dy}}{{dx}} = - \frac{{{x^{n - 1}}}}{{{a^n}}}.\frac{{{b^n}}}{{{y^{n - 1}}}} \cr & \therefore \,{\left( {\frac{{dy}}{{dx}}} \right)_{{\text{at }}\left( {a,\,b} \right)}} = - \frac{b}{a} = {\text{the slope of }}\frac{x}{a} + \frac{y}{b} = 2 \cr} $$
Hence, it touches the curve at $$\left( {a,\,b} \right)$$  whatever may be the value of $$n.$$

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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