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)=limh→0√|x+h|sin(x+h)−√|x|sin(x)h
But if I try to find the derivative at 0 I get:
f′(0)=limh→0√|h|sin(h)h=0
which is not true because the derivative DNE at 0
because:
f′(x)={√xcosx+sinx2√x, x>0√−xcosx−sinx2√−x 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,
limh→0√|h|sin(h)h=limh→0√|h|⋅limh→0sin(h)h=0⋅1.
As the limit exists, this is the value of the derivative.
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