Question

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
Answer :   it is at a constant distance from the origin
Solution :
$$\eqalign{ & \frac{{dx}}{{d\theta }} = a\left( { - \sin \theta + \sin \theta + \theta \cos \theta } \right) = a\theta \cos \theta \,......\left( 1 \right) \cr & \frac{{dy}}{{d\theta }} = a\left( {\cos \theta - \cos \theta + \theta \sin \theta } \right) = a\theta \sin \theta \,......\left( 2 \right) \cr} $$
Dividing (2) by (1), we get
$$\eqalign{ & \frac{{dy}}{{dx}} = \tan \theta \,\left( {{\text{slope of tangent}}} \right) \cr & \therefore {\text{Slope of normal }} = - \cot \theta \cr & \therefore {\text{Equation of normal is}} \cr & y - a\left( {\sin \theta - \theta \cos \theta } \right) = - \frac{{\cos \theta }}{{\sin \theta }}\left( {x - a\left( {\cos \theta + \theta \sin \theta } \right)} \right) \cr & \Rightarrow y\sin \theta - a{\sin ^2}\theta + a\sin \theta \cos \theta \cr & = - x\cos \theta + a{\cos ^2}\theta + a\theta \sin \theta \cos \theta \cr & \Rightarrow x\cos \theta + y\sin \theta = a \cr} $$
As $$\theta $$ varies inclination is not constant.
∴ (A) is not correct.
Clearly does not pass through (0, 0).
It's distance from origin $$ = \left| {\frac{a}{{\sqrt {{{\cos }^2}\theta + {{\sin }^2}\theta } }}} \right| = a$$
which is constant

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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