ONE MORE STEP TOWARD BETTER TOMORROW : CHAPTER 12- Worked Out Examples

Sunday, 3 August 2014

CHAPTER 12- Worked Out Examples

Example: 1

Determine the intervals in which the following functions are increasing or decreasing:

	(a) $f\left( x \right) = {x^3} - x$
	(b) $f\left( x \right) = {x^3} - 6{x^2} + 11x - 6$

Solution: 1

In this and subsequent questions where we are required to find out the intervals of increase/decrease, we first determine f ‘(x). $f(x)$ increases in all intervals where $f'(x) > 0$ and decreases in all intervals where $f'(x) < 0$ .

(a) $\,\,$ $f'\left( x \right) = 3{x^2} - 1$

Interval(s) of strict increase: $\,\,\,$ $f'\left( x \right) > 0$

	$\Rightarrow \,\,\,3{x^2} - 1 > 0$
	$\Rightarrow \,\,\,x < \dfrac{{ - 1}}{{\sqrt 3 }}{\rm{ or }}x > \dfrac{1}{{\sqrt 3 }}$

Interval(s) of strict decrease: $\,\,\,$ $f'\left( x \right) < 0$

	$\Rightarrow \,\,\,3{x^2} - 1 < 0$
	$\Rightarrow \,\,\,\dfrac{{ - 1}}{{\sqrt 3 }} < x < \dfrac{1}{{\sqrt 3 }}$

Therefore, $f(x)$ increases in $\left( { - \infty ,\dfrac{{ - 1}}{{\sqrt 3 }}} \right) \cup \left( {\dfrac{1}{{\sqrt 3 }},\infty } \right)$ and decreases in $\left( {\dfrac{{ - 1}}{{\sqrt 3 }},\dfrac{1}{{\sqrt 3 }}} \right)$ . The graph for $f(x)$ confirms this: (to plot the graph, the knowledge of roots of $f(x)$ helps, which is easy to obtain for this example; ${x^3} - x = 0 \Rightarrow x = 0, \pm 1$ )

Solution: 1

$f'\left( x \right) = 3{x^2} - 12x + 11$

The roots of f ‘(x) are $\dfrac{{12 \pm \sqrt {12} }}{6} = \dfrac{{6 \pm \sqrt 3 }}{3} = 2 \pm \dfrac{1}{{\sqrt 3 }}$

Interval(s) of strict increase: $\,\,\,$ $f'\left( x \right) > 0$

	$\Rightarrow \,\,\,3{x^2} - 12x + 11 > 0$
	$\Rightarrow \,\,\,x < 2 - \dfrac{1}{{\sqrt 3 }}{\rm{ or }}x > 2 + \dfrac{1}{{\sqrt 3 }}$

Interval(s) of strict decrease: $\,\,\,$ $f'\left( x \right) < 0$

	$\Rightarrow \,\,\,3{x^2} - 12x + 11 < 0$
	$\Rightarrow \,\,\,2 - \dfrac{1}{{\sqrt 3 }} < x < 2 + \dfrac{1}{{\sqrt 3 }}$

$f(x)$ can be factorised as $(x - 1)(x - 2)(x - 3)$ so that the roots of $f(x)$ are $x = 1,2,3$ . The graph for $f(x)$ is approximately sketched below:

Example: 2

Determine the values of $x$ for which $f(x) = {x^x},x > 0$ is increasing or decreasing.

Solution: 2

To find $f'(x)$ , we first take the logarithm of both sides of the given equation:

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

Differentiating both sides, we get:

	$\dfrac{1}{{f\left( x \right)}} \cdot f'\left( x \right) = x \cdot \dfrac{1}{x} + \ln x \cdot 1$
	$= 1 + \ln x$
	$\Rightarrow \,\,\,\, f'\left( x \right) = {x^x}\left( {1 + \ln x} \right)$

Interval(s) of strict increase: $\,\,\,$ $f'\left( x \right) > 0$

	$\Rightarrow \,\,\,\, {x^x}\left( {1 + \ln x} \right) > 0$
	$\Rightarrow \,\,\,\, 1 + \ln x > 0$
	$\Rightarrow \,\,\,\, x > {e^{ - 1}} = \dfrac{1}{e}$

Interval(s) of strict decrease: $\,\,\,$ $f'\left( x \right) < 0$

	$\Rightarrow \,\,\,\, {x^x}\left( {1 + \ln x} \right) < 0$
	$\Rightarrow \,\,\,\, x < \dfrac{1}{e}$

To plot the graph of $f(x)$ , notice that $\mathop {\lim }\limits_{x \to 0} f\left( x \right) = \mathop {\lim }\limits_{x \to 0} {x^x}$

$= \mathop {\lim }\limits_{x \to 0} {e^{x\ln x}} = {e^{\mathop {\lim }\limits_{x \to 0} .x\ln x}} = {e^0} = 1.$

Also,

$\mathop {\lim }\limits_{x \to \infty } \left( {f\left( x \right)} \right) = \infty .$

$f(x)$ decreases in ${\rm{(0, 1/e) }}$ and increases in $\left( {\dfrac{1}{e},\infty } \right)$ .

Example: 3

Separate the interval $[0,\pi /2]$ into sub-intervals in which $f\left( x \right) = {\sin ^4}x + {\cos ^4}x$ is increasing or decreasing.

Solution: 3

	$f'\left( x \right) = 4{\sin ^3}x\cos x - 4{\cos ^3}x\sin x$
	$= 4\sin x\cos x\left( {{{\sin }^2}x - {{\cos }^2}x} \right)$
	$= 2\sin 2x\left( { - \cos 2x} \right)$
	$= - \sin 4x$

We now need to consider the sign of $f'(x)$ in the interval [0, p/2].

Interval (s) of strict increase: $\,\,\,$ $f'\left( x \right) > 0$

	$\Rightarrow \,\,\,\, - \sin 4x > 0$
	$\Rightarrow \,\,\, \,\sin 4x < 0$
	$\Rightarrow \,\,\,\, \pi < 4x < 2\pi \,\,\,\left( \begin{array}{l} {\rm{This\, range\, of\, 4}}x\,{\rm{ will\, ensure \,}}\\ {\rm{that\, }}x\;{\rm{itself \,lies\, in\, }}\left[ {0,\dfrac{\pi }{2}} \right] \end{array} \right)$
	$\Rightarrow \,\,\,\, \dfrac{\pi }{4} < x < \dfrac{\pi }{2}$

Interval(s) strict decrease:

	$f'\left( x \right) < 0$
	$\Rightarrow \,\,\,\, - \sin 4x < 0$
	$\Rightarrow \,\,\,\, 0 < 4x < \pi$
	$\Rightarrow \,\,\,\, 0 < x < \pi /4$

Therefore, $f(x)$ decreases in $[0,\pi /4]$ and increases in . The minimum value in $[0,\pi /2]$ is at $x = \dfrac{\pi }{4}$ equal to $f\left( x \right) = \dfrac{1}{2}$ and the maximum value is at $x = 0$ or $x = \dfrac{\pi }{2}$ equal to $f(x) = 1$ . The graph is approximately sketched below:

Sunday, 3 August 2014

CHAPTER 12- Worked Out Examples

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