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Compare (Posted on 2011-07-26) Difficulty: 4 of 5
Which is bigger: ln(1.01) or 2/201?
Pen and paper only.

No Solution Yet Submitted by Ady TZIDON    
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Solution Solution Comment 4 of 4 |
ln(1.01) and 2/201 can be thought of as evaluating ln(1 + 1/x) and 1/(x + 1/2) at x=100.  Then if ln(1 + 1/x) is greater than 1/(x + 1/2) then [ln(1 + 1/x)]/[1/(x + 1/2)] > 1.

lim x->inf  [ln(1 + 1/x)]/[1/(x + 1/2)] can be evaluated by first applying L'Hopital's rule (and simplify a bit) to get 
lim x->inf(x^2+x+1/4)/(x^2+x)
This is pretty easy to get the limit equals 1.

d/dx [ln(1 + 1/x)]/[1/(x + 1/2)] = (x + 1/2)*ln(1 + 1/x) comes out pretty straightforward as
ln(1 + 1/x) - (x + 1/2)/(x^2+x)
This can be manipulated into ln(1 + 1/x) - 1 - 4/(x^2+x)
Now, for any x>=1, 1<1+1/x<=2<e, which bounds ln(1 + 1/x) between 0 and 1.  The important consequence of this is that d/dx will then always be negative for x>=1.

So now on [1,inf) we have [ln(1 + 1/x)]/[1/(x + 1/2)] is continuously decreasing and has a horizontal asymptote of 1.  This means that the function must be greater than 1 for all x>=1.

So then f(100) = ln(1.01)/(2/201) > 1, which means that ln(1.01) > 2/201.

  Posted by Brian Smith on 2022-12-22 22:55:18
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