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