Evaluate the limit as x approaches 1 for (1/lnx) − (1/(x-1))
Thanks
Let's look at it as
lim x->1 of { [x - 1 - ln(x)] / [ln(x)*(x-1)] }
The derivative of the numerator is 1 - 1/x.
The derivative of the denominator is
ln(x) + (x-1)/x = ln(x) + 1 - 1/x.
Both of these still go to zero when x goes to 1,
so we will apply L'Hopital's Rule a second time,
trying to find
lim x->1 { (1 - 1/x) / [ln(x) + 1 - 1/x] }.
The derivative of the numerator is 1/x^2.
The derivative of the denominator is 1/x + 1/x^2.
So the limit should be 1/2.
To check this, I'll let x be 1.00001. Then
(1/ln(1.00001)) - 1/(0.00001)
= 0.49999851 by the Google calculator,
so my answer of 1/2 looks good!
Lt (x→1) [1/lnx – 1/(x-1)]
Using L-Hospital rule
Lt (x→1) [0/1/x – (-1/(x-1)²]
Lt (x→1) [1/(x-1)²
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Verified answer
Let's look at it as
lim x->1 of { [x - 1 - ln(x)] / [ln(x)*(x-1)] }
The derivative of the numerator is 1 - 1/x.
The derivative of the denominator is
ln(x) + (x-1)/x = ln(x) + 1 - 1/x.
Both of these still go to zero when x goes to 1,
so we will apply L'Hopital's Rule a second time,
trying to find
lim x->1 { (1 - 1/x) / [ln(x) + 1 - 1/x] }.
The derivative of the numerator is 1/x^2.
The derivative of the denominator is 1/x + 1/x^2.
So the limit should be 1/2.
To check this, I'll let x be 1.00001. Then
(1/ln(1.00001)) - 1/(0.00001)
= 0.49999851 by the Google calculator,
so my answer of 1/2 looks good!
Lt (x→1) [1/lnx – 1/(x-1)]
Using L-Hospital rule
Lt (x→1) [0/1/x – (-1/(x-1)²]
Lt (x→1) [1/(x-1)²