Monday, 16 September 2013

calculus - Mysterious limit of a function



this is driving me crazy. I need to solve this:



enter image description here



Here's the riddle: Since it's (0/0), I do L'Hôpital's rule, which means I get to:



enter image description here




But the limit of this = 0 (after one more use of L'Hôpital's rule). And that is not the correct answer.



HOWEVER, if I just do the derivative of the integral, but LEAVE the denominator as is, then I get this:



enter image description here



And after some more use of L'Hôpital's rule, this actually comes out to be (-5) - which is supposed to be the correct answer.



So I don't understand why when I use L'Hôpital's rule on the numerator alone - it works, but if I use it on both numerator and denominator (which is how you're supposed to..) - it doesn't.




Would appreciate the solution of this "mystery" :)


Answer




this actually comes out to be (-5) - which is supposed to be the correct answer.




It isn't.



If we Taylor-expand the integrand, we get




x0e5t21tdt=x05t2+O(t4)tdt=x05t+O(t3)dt=52x2+O(x4).



The denominator is



1+2x1=(1+xx22+O(x3))1=x+O(x2),



so the limit is indeed 0.


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