I stumbled on the following limit in a calculus textbook today:
limx→0ln(x)cot(x)
According to the book's solutions and Mathematica, this limit exists and is equal to 0. I can see why 0 is obtained using l'Hôpital's rule twice:
...=limx→0(1x)−csc2(x)=−limx→0sin2(x)x=−limx→02sin(x)cos(x)1=2sin(0)cos(0)=0
If I recall correctly, l'Hôpital's rule is applicable when we have:
limx→af(x)g(x)
even if f and g are not derivable at precisely a, so there should be no issue in using it on the above limit.
However, I can't reconcile the fact that ln(x) is defined over ]0,+∞[ (and usually, only limx→0+ln(x) exists) with the fact that the above limit exists (both as x→0+ and as x→0−).
It seems to me that only
limx→0+ln(x)cot(x)
should exist and thus the "bilateral limit" (with x→0) does not exist since the limit with x→0− doesn't.
Is there something I am missing?
Answer
Yes you are right, since lnx is defined for x>0 the limit for x→0− is meaningless and only
limx→0+lnxcotx
can be considered.
Note that this doesn't mean in general that the limit considered exists.
For example
limx→0+lnxsin1x
can be considered but does not exist.
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