Question

The total number of parallel tangents of $${f_1}\left( x \right) = {x^2} - x + 1$$     and $${f_2}\left( x \right) = {x^3} - {x^2} - 2x + 1$$      is :

A. 2
B. 0
C. 1
D. infinite  
Answer :   infinite
Solution :
Here,
$$\eqalign{ & {f_1}\left( x \right) = {x^2} - x + 1\,\,{\text{and }}{f_2}\left( x \right) = {x^3} - {x^2} - 2x + 1 \cr & {\text{or }}{f_1}'\left( {{x_1}} \right) = 2{x_1} - 1\,\,{\text{and }}{f_2}'\left( {{x_2}} \right) = 3x_2^2 - 2{x_2} - 2 \cr} $$
Let the tangents drawn to the curves $$y = {f_1}\left( x \right)$$   and $$y = {f_2}\left( x \right)$$   at $$\left( {{x_1},\,{f_1}\left( {{x_1}} \right)} \right)$$   and $$\left( {{x_2},\,{f_2}\left( {{x_2}} \right)} \right)$$   be parallel. Then
$$\eqalign{ & 2{x_1} - 1 = 3x_2^2 - 2{x_2} - 2 \cr & {\text{or }}2{x_1} = \left( {3x_2^2 - 2{x_2} - 1} \right) \cr} $$
So, which is possible for infinite numbers of ordered pairs. So, there are infinite solutions.

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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