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1

### AIEEE 2009

Let $$f\left( x \right) = x\left| x \right|$$ and $$g\left( x \right) = \sin x.$$
Statement-1: gof is differentiable at $$x=0$$ and its derivative is continuous at that point.
Statement-2: gof is twice differentiable at $$x=0$$.
A
Statement-1 is true, Statement-2 is true; Statement-2 is not a correct explanation for Statement-1.
B
Statement-1 is true, Statement-2 is false
C
Statement-1 is false, Statement-2 is true
D
Statement-1 is true, Statement-2 is true Statement-2 is a correct explanation for Statement-1

## Explanation

Given that $$f\left( x \right) = x\left| x \right|\,\,$$ and $$\,\,g\left( x \right) = \sin x$$

So that go

$$f\left( x \right) = g\left( {f\left( x \right)} \right)$$

$$= g\left( {x\left| x \right|} \right) = \sin x\left| x \right|$$

$$= \left\{ {\matrix{ {\sin \left( { - {x^2}} \right),} & {if\,\,\,x < 0} \cr {\sin \left( {{x^2}} \right),} & {if\,\,\,x \ge 0} \cr } } \right.$$

$$= \left\{ {\matrix{ { - \sin \,{x^2},} & {if\,\,\,x < 0} \cr {\sin \,\,{x^2},} & {if\,\,\,x \ge 0} \cr } } \right.$$

$$\therefore$$ $$\left( {go\,f} \right)'\,\,\left( x \right) = \left\{ {\matrix{ { - 2x\,\,\cos \,{x^2},\,\,\,\,if\,\,\,\,x < 0} \cr {2x\,\cos \,{x^2},\,\,\,if\,\,\,\,x \ge 0} \cr } } \right.$$

Here we observe

$$L\left( {gof} \right)'\left( 0 \right) = 0 = R\left( {gof} \right)'\left( 0 \right)$$

$$\Rightarrow$$ go $$f$$ is differentiable at $$x=0$$

and $$\left( {go\,f} \right)'$$ is continuous at $$x=0$$

Now $$\left( {go\,f} \right)''\left( x \right) = \left\{ {\matrix{ { - 2\cos {x^2} + 4{x^2}\sin {x^2},x < 0} \cr {2\cos {x^2} - 4{x^2}\sin {x^2},x \ge 0} \cr } } \right.$$

Here $$L\left( {gof} \right)''\left( 0 \right) = - 2$$ and $$R\left( {go\,f} \right)''\left( 0 \right) = 2$$

As $$L{\left( {go\,f} \right)^{''}}\left( 0 \right) \ne R\left( {go\,f} \right)''\,\,\left( 0 \right)$$

$$\Rightarrow go\,f\left( x \right)$$ is not twice differentiable at $$x=0.$$

$$\therefore$$ Statement - $$1$$ is true but statement $$-2$$ is false.
2

### AIEEE 2008

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 $$\sqrt {{p \over 3}}$$ and maxima at $$-\sqrt {{p \over 3}}$$
B
The cubic has minima at $$-\sqrt {{p \over 3}}$$ and maxima at $$\sqrt {{p \over 3}}$$
C
The cubic has minima at both $$\sqrt {{p \over 3}}$$ and $$-\sqrt {{p \over 3}}$$
D
The cubic has maxima at both $$\sqrt {{p \over 3}}$$ and $$-\sqrt {{p \over 3}}$$

## Explanation

Let $$y = {x^3} - px + q$$

$$\Rightarrow {{dy} \over {dx}} = 3{x^2} - p$$

For $${{dy} \over {dx}} = 0 \Rightarrow 3{x^2} - p = 0$$

$$\Rightarrow x = \pm \sqrt {{p \over 3}}$$

$${{{d^2}y} \over {d{x^2}}} = 6x$$

$${\left. {{{{d^2}y} \over {d{x^2}}}} \right|_{x = \sqrt {{p \over 3}} }} = + ve\,\,\,\,$$ and

$$\,\,\,\,\,\,\,\,\,\,$$ $${\left. {\,\,\,{{{d^2}y} \over {d{x^2}}}} \right|_{x = - \sqrt {{p \over 3}} }} = - ve$$

$$\therefore$$ $$y$$ has ninima at $$x = \sqrt {{p \over 3}}$$

and maxima at $$x = - \sqrt {{p \over 3}}$$
3

### AIEEE 2008

How many real solutions does the equation
$${x^7} + 14{x^5} + 16{x^3} + 30x - 560 = 0$$ have?
A
$$7$$
B
$$1$$
C
$$3$$
D
$$5$$

## Explanation

Let $$f\left( x \right) = {x^7} + 14{x^5} + 16{x^3} + 30x - 560$$

$$\Rightarrow f'\left( x \right) = 7{x^6} + 70{x^4} + 48{x^2} + 30 > 0,\,\forall x \in R$$

$$\Rightarrow f$$ is an increasing function on $$R$$

Also $$\mathop {\lim }\limits_{x \to \infty } \,\,f\left( x \right) = \infty$$ and $$\mathop {\lim }\limits_{x \to - \infty } \,\,f\left( x \right) = - \infty$$

$$\Rightarrow$$ The curve $$y = f\left( x \right)$$ crosses $$x$$-axis only once.

$$\therefore$$ $$f\left( x \right) = 0$$ has exactly one real root.
4

### AIEEE 2007

If $$p$$ and $$q$$ are positive real numbers such that $${p^2} + {q^2} = 1$$, then the maximum value of $$(p+q)$$ is
A
$${1 \over 2}$$
B
$${1 \over {\sqrt 2 }}$$
C
$${\sqrt 2 }$$
D
$$2$$

## Explanation

Given that $${p^2} + {q^2} = 1$$

$$\therefore$$ $$p = \cos \theta$$ and $$q = \sin \theta$$

Then $$p+q$$ $$= \cos \theta + \sin \theta$$

We know that

$$- \sqrt {{a^2} + {b^2}} \le a\cos \theta + b\sin \theta \le \sqrt {{a^2} + {b^2}}$$

$$\therefore$$ $$- \sqrt 2 \le \cos \theta + \sin \theta \le \sqrt 2$$

Hence max. value of $$p + q$$ is $$\sqrt 2$$

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