91.
The normal to the curve $$y\left( {x - 2} \right)\left( {x - 3} \right) = x + 6$$ at the point where the curve intersects the $$y$$ -axis passes through the point:
A
$$\left( {\frac{1}{2},\frac{1}{3}} \right)$$
B
$$\left( { - \frac{1}{2}, - \frac{1}{2}} \right)$$
Clearly, domain of the function is $$\left[ {2,\,4} \right].$$
$$\eqalign{
& {\text{Now, }}f'\left( x \right) = \frac{1}{{\sqrt {x - 2} }} - \frac{1}{{2\sqrt {4 - x} }} \cr
& {\text{or, }}f'\left( x \right) = 0 \cr
& {\text{or, }}\sqrt {x - 2} = 2\sqrt {4 - x} \cr
& {\text{or, }}x - 2 = 16 - 4x \cr
& {\text{or, }}x = \frac{{18}}{5} \cr
& {\text{Now, }}f\left( 2 \right) = \sqrt 2 , \cr
& f\left( {\frac{{18}}{5}} \right) = 2\sqrt {\frac{{18}}{5} - 2} + \sqrt {4 - \frac{{18}}{5}} = \sqrt {10} , \cr
& f\left( 4 \right) = 2\sqrt 2 \cr} $$
Hence, range of the function is $$\left[ {\sqrt 2 ,\,\sqrt {10} } \right].$$
Also, here $$x = \left( {\frac{{18}}{5}} \right)$$ is the point of global maxima.
95.
The eccentricity of an ellipse whose centre is at the origin is $$\frac{1}{2}.$$ If one of its directices is $$x = - 4$$ then the equation of the normal to it at $$\left( {1,\frac{3}{2}} \right)$$ is:
96.
Let $$f\left( x \right) = {e^x}\sin \,x$$ be the equation of a curve. If at $$x = a,\,0 \leqslant a \leqslant 2\pi ,$$ the slope of the tangent is the maximum then the value of $$a$$ is :
$$f'\left( x \right) = {e^x}\left( {\sin \,x + \cos \,x} \right).$$ Therefore, the slope $$m$$ of the tangent is given by $$m = {e^x}\left( {\sin \,x + \cos \,x} \right).$$
Now, $$\frac{{dm}}{{dx}} = {e^x}\left\{ {\sin \,x + \cos \,x + \cos \,x - \sin \,x} \right\} = 2{e^x}.\cos \,x$$
and $$\frac{{{d^2}m}}{{d{x^2}}} = 2{e^x}\left( {\cos \,x - \sin \,x} \right)$$
At $$x=a,\,m$$ is the maximum
$$\eqalign{
& \therefore \,\,{\left. {\frac{{dm}}{{dx}}} \right)_{x = a}} = 0,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\left. {\frac{{{d^2}m}}{{d{x^2}}}} \right)_{x = a}} < 0 \cr
& {\left. {\frac{{dm}}{{dx}}} \right)_{x = a}} = 0\,\,\,\,\,\, \Rightarrow 2{e^a}\cos \,a = 0 \cr
& \Rightarrow a = \frac{\pi }{2},\,\frac{{3\pi }}{2}{\text{ because }}0 \leqslant a \leqslant 2\pi \cr
& {\left. {\frac{{{d^2}m}}{{d{x^2}}}} \right)_{x = a}} < 0\,\,\,\,\,\,\, \Rightarrow 2{e^a}\left( {\cos \,a - \sin \,a} \right) < 0 \cr
& {\text{This is satisfied by }}a = \frac{\pi }{2} \cr} $$
97.
If the tangent to the curve, $$y = {x^3} + ax - b$$ at the point (1, -5) is perpendicular to the line, $$ - x + y + 4 = 0, $$ then which one of the following points lies on the curve?
$$y = {x^3} + ax - b$$
Since, the point (1, -5) lies on the curve.
$$\eqalign{
& \Rightarrow 1 + a - b = - 5 \cr
& \Rightarrow a - b = - 6\,.......\left( 1 \right) \cr
& {\text{Now, }}\frac{{dy}}{{dx}} = 3{x^2} + a \Rightarrow {\left( {\frac{{dy}}{{dx}}} \right)_{{\text{at }}x = 1}} = 3 + a \cr} $$
Since, required line is perpendicular to $$y = x - 4,$$ then slope of tangent at the point $$P\left( {1, - 5} \right) = - 1$$
$$\therefore 3 + a = - 1 \Rightarrow a = - 4 \Rightarrow b = 2$$ the equation of the curve is $$y = {x^3} - 4x - 2$$
$$ \Rightarrow \left( {2, - 2} \right)$$ lies on the curve.
98.
Suppose the cubic $${x^3} - px + q$$ has three distinct real roots where $$p > 0$$ and $$q > 0.$$ Then which one of the following holds?
A
The cubic has minima at $$\sqrt {\frac{p}{3}} $$ and maxima at $$ - \sqrt {\frac{p}{3}} $$
B
The cubic has minima at $$ - \sqrt {\frac{p}{3}} $$ and maxima at $$\sqrt {\frac{p}{3}} $$
C
The cubic has minima at both $$\sqrt {\frac{p}{3}} $$ and $$ - \sqrt {\frac{p}{3}} $$
D
The cubic has maxima at both $$\sqrt {\frac{p}{3}} $$ and $$ - \sqrt {\frac{p}{3}} $$
Answer :
The cubic has minima at $$\sqrt {\frac{p}{3}} $$ and maxima at $$ - \sqrt {\frac{p}{3}} $$
$$\eqalign{
& {\text{Let }}y = {x^3} - px + q \Rightarrow \frac{{dy}}{{dx}} = 3{x^2} - p \cr
& {\text{For }}\frac{{dy}}{{dx}} = 0 \Rightarrow 3{x^2} - p = 0 \Rightarrow x = \pm \sqrt {\frac{p}{3}} \cr
& \frac{{{d^2}y}}{{d{x^2}}} = 6x \cr
& {\left. {\frac{{{d^2}y}}{{d{x^2}}}} \right|_{x = \sqrt {\frac{p}{3}} }} = + ve{\text{ and }}{\left. {\frac{{{d^2}y}}{{d{x^2}}}} \right|_{x = - \sqrt {\frac{p}{3}} }} = - ve \cr
& \therefore y{\text{ has minima at }}x = \sqrt {\frac{p}{3}} {\text{ and maxima at }}x = - \sqrt {\frac{p}{3}} \cr} $$
99.
Let $$f\left( x \right) = {x^2} + \frac{1}{{{x^2}}}$$ and $$g\left( x \right) = x - \frac{1}{x},\,x \in R - \left\{ { - 1,0,1} \right\}.$$ If $$h\left( x \right) = \frac{{f\left( x \right)}}{{g\left( x \right)}},\,$$ then the local minimum value of $$h\left( x \right)$$ is:
100.
Let $$f\left( x \right)$$ be a function defined as below:
\[f\left( x \right) = \left\{ \begin{array}{l}
\sin \left( {{x^2} - 3x} \right),\,x \le 0\\
6x + 5{x^2},\,x > 0
\end{array} \right.\]
Then at $$x = 0,\,f\left( x \right)$$