this is driving me crazy. I need to solve this:
Here's the riddle: Since it's (0/0), I do L'Hôpital's rule, which means I get to:
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:
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
∫x0e−5t2−1tdt=∫x0−5t2+O(t4)tdt=∫x0−5t+O(t3)dt=−52x2+O(x4).
The denominator is
√1+2x−1=(1+x−x22+O(x3))−1=x+O(x2),
so the limit is indeed 0.
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