5. Continuity and Differentiation
easy

Rolle's theorem is not applicable to the function $f(x) = |x|$ defined on $ [-1, 1] $ because

A

$f $ is not continuous on $ [ -1, 1]$

B

$f$  is not differentiable on $ (-1,1)$

C

$f( - 1) \ne f(1)$

D

$f( - 1) = f(1) \ne 0$

Solution

(b) $f(x) = \left\{ \begin{array}{l} – x,\,{\rm{when\,\, -1}} \le x < 0\\{\rm{ }}x,\;{\rm{when\,\,}}\;{\rm{0}} \le x \le {\rm{1}}\end{array} \right.$

Clearly $f( – 1) = | – 1| = 1 = f(1)$

But $Rf'(0) = \mathop {\lim }\limits_{h \to 0}\frac{{f(0 + h) – f(0)}}{h} = \mathop {\lim }\limits_{h \to 0} \frac{{|h|}}{h}$

$ = \mathop {\lim }\limits_{h \to 0} \frac{h}{h} = 1$

$Lf'(0) = \mathop {\lim }\limits_{h \to 0} \frac{{f(0 – h) – f(0)}}{h} = \mathop {\lim }\limits_{h \to 0} \frac{{| – h|}}{{ – h}}$

$ = \mathop {\lim }\limits_{h \to 0} \frac{h}{{ – h}} = – 1$

$\therefore Rf'(0) \ne Lf'(0)$

Hence it is notdifferentiable on $( – 1,\,\,1)$.

Standard 12
Mathematics

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