Sunday, August 25, 2019

real analysis - How do i evaluate this limit :limxto0fracexx1x² without using Taylor expansion




I would like to evaluate this limit without using Taylor expansion:




limx0exx1x2

.



Note: by Taylor Expansion i have got :12 .



Thank u for any help .!!!!


Answer



METHODOLOGY 1: Use L'Hospital's Rule Successively



Repeated use of L'Hospital's Rule reveals




limx0exx1x2=limx0ex12x=limx012ex=12






METHODOLOGY 2: Integral representation of the numerator




Note that we can write the numerator as



exx1=x0t0esdsdt=x0xsesdtds=x0(xs)esds



Now, we can use the Mean-Value-Theorem for integrals to reveal




exx1=es(x)x0(xs)ds=12x2es(x)



for some value of s(x)(0,x).



Finally, exploiting the continuity of the exponential function yield the coveted limit



limx0exx1x2=limx012x2es(x)x2=12



as expected!


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