Prove that: \lim_{x\rightarrow0}\frac{\ln(\cos x)}{\ln(1-\frac{x^2}{2})}=1 without L'Hopital's rule.

David Young

David Young

Answered question

2021-12-30

Prove that:
limx0ln(cosx)ln(1x22)=1
without LHopitals rule.

Answer & Explanation

Esta Hurtado

Esta Hurtado

Beginner2021-12-31Added 39 answers

Using ln(1y)=n=1ynn we get immediately
ln(1x22)=x22+O(x4),and also, after observing that
cosx=1x22+O(x4),lncosx=x22+O(x4)
Therefore
ln(cosx)ln(1x22)=1+O(x2)

Gerald Lopez

Gerald Lopez

Beginner2022-01-01Added 29 answers

ln(cosx)ln(1x22)=ln(1+cosx1)cosxx22ln(1x22)sin2x2x241
Vasquez

Vasquez

Expert2022-01-09Added 669 answers

On OP's request I am converting my comment into an answer. The limit in question is easily solved if one rewrites the given expression as
log(1+cosx1)cosx1cosx1x2x2(x22)(x22)log(1x22)
And then the limit is easily seen to be 1(12)(2)1=1.
Other limits in your question (after the fold) don't have any problem as we can just plug x=0 to evaluate them (the functions concerned are continuous at 0).

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