Sunday, February 3, 2019

limits - How to prove that xlnleft(fracex+1ex1right) tends to 0 as xto0,infty




How could it be shown that limx0[xln(ex+1ex1)]=limx[xln(ex+1ex1)]=0?




Note that when x=0, we have x=0 (obviously) and ln(ex+1ex1)=ln(20) which is indeterminate and when x, we have x (obviously) and ln(ex+1ex1)=ln(1+2ex1)ln1=0. So it is not possible to just multiply both.




I can't see L'Hopital working as the fraction in ln expands to a sum, not a fraction.



Here's a plot of the function in Desmos.



Any approaches?


Answer



xln(ex+1ex1)=xln(ex+1)xln(ex1) and it is clear that xln(ex+1)0 as x0. To find the limit of xln(ex1) apply L'Hopital Rule to ln(ex1)1/x and use the fact that x2ex10 as x0 (by Another application of L'Hopital Rule). For limit as x use the fact that ln(ex+1)(ex1)=ln(1+2ex1) and then use the fact that ln(1+x) behaves like x as x0.


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