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)=limh0|x+h|sin(x+h)|x|sin(x)h



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




f(0)=limh0|h|sin(h)h=0



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



f(x)={xcosx+sinx2x,     x>0xcosxsinx2x   x<0



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,



limh0|h|sin(h)h=limh0|h|limh0sin(h)h=01.




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


No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find limh0sin(ha)h without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...