Saturday, 19 July 2014

calculus - Derivative by definition



I'm trying to find the derivative by definition of the following function:




f(x)=|x|sin(x)



I know that by definition:



f(x)=lim



But if I try to find the derivative at 0 I get:




f'(0)=\lim_{h\to0}\frac{\sqrt{|h|}\sin(h)}{h}=0



which is not true because the derivative DNE at 0
because:



f'(x)=\begin{cases} \sqrt{x} \cos x+\frac{\sin x}{2\sqrt{x}}, \ \ \ \ \ x>0 \\ \sqrt{-x} \cos x-\frac{\sin x}{2\sqrt{-x}}\ \ \ x<0 \\ \end{cases}



So how can it be that the derivative exists only when it is calculated by definition?


Answer



You shouldn't say that the derivative does not exist.



Indeed,



\lim_{h\to0}\frac{\sqrt{|h|}\sin(h)}{h}=\lim_{h\to0}\sqrt{|h|}\cdot\lim_{h\to0}\frac{\sin(h)}{h}=0\cdot1.




As the limit exists, this is the value of the derivative.


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