Wednesday, 1 May 2013

real analysis - How to prove that limlimitsxtoinftyextextarccot(x)=infty?




How to prove that
limxexarccot(x)=?





I already figured that ddx[arrcot(x)]=ddx[arctan(1x)]=1x2+1. Now I wanted to use L'Hospitals rule after doing some algebra:limxexarccot(x)=limxex1arccot(x)=limxex1(x2+1)arccot2(x)

using it twice didn't work out aswell, what am I supposed to do?


Answer



limxarccotxex=limx11+x2ex=limxex1+x2



=limx1+x+x22+x33!+O(x4)1+x2



Divide numerator & denominator by x2


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