Inspired by Jake Lai

Algebra Level 2

It is well known that ln 2 < ln 3 \ln 2 < \ln 3 . Which of the following is bigger:

[ ln ( ln 2 ) ] 2 or [ ln ( ln 3 ) ] 2 ? [ \ln ( \ln 2 ) ] ^2 \text { or } [ \ln ( \ln 3) ]^2 ?


Inspiration, see solution .

[ ln ( ln 3 ) ] 2 [\ln ( \ln 3 ) ] ^2 [ ln ( ln 2 ) ] 2 [\ln ( \ln 2 ) ] ^2

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2 solutions

Otto Bretscher
Jan 6, 2016

Using trapezoids, we see that ln 2 = 1 2 d t t < 3 4 \ln2=\int_{1}^{2}\frac{dt}{t}<\frac{3}{4} and ln 3 < 4 3 \ln3<\frac{4}{3} . Thus ln 3 < ( ln 2 ) 1 \ln3<(\ln2)^{-1} and ln ln 3 < ln ln 2 \ln\ln3<-\ln\ln2 . Since ln ln 3 > 0 \ln\ln3>0 , we can square both sides to find that ( ln ln 3 ) 2 < ( ln ln 2 ) 2 (\ln\ln3)^2<(\ln\ln2)^2

Oh wow, that's a very nice way to show it!

Calvin Lin Staff - 5 years, 5 months ago

This has got to be one of the better solutions on this site.

Nicky S. - 5 years, 5 months ago

ln(ln(2))=-.3665... ln(ln(3))=.094...

Prasit Sarapee - 5 years, 5 months ago

3/4 and 4/3?

Soner Karaca - 5 years, 5 months ago

Wow. BRILLIANT!

Altan-Ulzii Chuluun - 1 year, 11 months ago
Aayush Patni
Jan 5, 2016

This solution was based on the assumption that ln \ln refers to log \log base 10.


0 < ln(2)<ln(3) < 1 Let ln(2)=a

ln(3)=b

Then ln(a)>ln(b) ( as ln(x) is decreasing function b/w 0 and 1)

Thus ln(a)^2>ln(b)^2

Wrong. ln x is strictly increasing in its entire domain and btw ln(3) >1.

Rishabh Jain - 5 years, 5 months ago

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Sorry. Silly mistake. Took it as log. My bad

Aayush Patni - 5 years, 5 months ago

Ln is always increasing. In fact its because a<b<0 => a^2 > b^2 > 0 that let as conclude. ( in this case ln(ln(2)) < ln(ln(3))<0 => [ ln(ln(2)) ]^2 > [ ln(ln(3)) ]^2

Sam Lee - 5 years, 5 months ago

In(In 2)^2 = 0.13433 and In (In 3)^2 = 0.00884 so In(In 2)^2 is greater than In(In3)^2

alicia albia - 11 months, 2 weeks ago

For those who are doing it with calculators. Quizzes aren't done by calculators, they test your thinking and numerical "analysis" not computing. So please stop computing.

Laxmi Narayan Bhandari Xth B - 8 months, 3 weeks ago

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