Prove the equation $\ln(x) = \frac1 {x-1}$ has exactly 2 real solutions. 
Prove the equation $\ln(x) = \frac1 {x-1}$ has exactly 2 real solutions.

Hello all. I thought of defining the function $f(x)=x-e^{\frac 1 {x-1}}$ and showing it has only 2 single roots, though I am not sure on how to show it and I understand it is better to prove using Lagrange's theorem.
Would be happy to get some help on that question, thanks in advance :)
 A: Consider the function $f$ defined on $(0,1)\cup(1,\infty)$ by $$f(x) = (x-1)\ln x\,.$$
We have 
$\lim_{x\to 0^+} f(x) = +\infty$, $\lim_{x\to 1^-} f(x)=\lim_{x\to 1^+} f(x) = 0$, and $\lim_{x\to +\infty} f(x) =+\infty$. Further, $f$ is differentiable twice on its domain, with
$$
f''(x)  = \frac{x+1}{x^2} > 0
$$
and
$$
f'(x)  = 1+\ln x - \frac{1}{x}
$$
which has limit $0$ at $x=1$ (on both sides), $-\infty$ at $0^+$ and $+\infty$ at $\infty$.Combining the two (the limits of $f'$ and the fact that $f'$ is strictly increasing on $(0,1)$, and strictly increasing on $(1,\infty)$), $f' <0$ on $(0,1)$ and $f'>0$ on $(1,\infty)$, i.e. you have strict monotonicity of $f$ on both parts of its domain. 
From this last thing, along with the limits of $f$, you get the result: $f$ takes the value $1$ exactly once on $(0,1)$, and exactly once on $(1,\infty)$. 
A: HINT
Consider and study the function $f(x)=(x-1)\log x-1$ using derivatives and IVT to prove that we have exactly 2 solutions for $f(x)=0$.
A: Consider $f(x)=ln(x)- \frac{1}{1-x}$ and then differentiate it once and look at how many times this function changes monotony
A: Start with this:  On $(1,\infty)$, $x\mapsto \ln(x)$ is strictly increasing and increases without bound.  On the same interval $x \mapsto 1/(x-1)$ decreases from $+\infty$ to $0$.  Now apply the intermediate value theorem to see you have at least one root on $(1,\infty)$.  The monotonicities can be used to see the root is unique.  Now deal similarly on $(0,1)$.  
A: Without using a study function, you can look over $]0,1[$ and $]1,+\infty[$. 
On both segments, $x\mapsto \frac{1}{x-1}$ and $x\mapsto \ln(x)$ are continuous respectively strictly decreasing and increasing : they can have on both segment at most one intersection point. Now looking on the values near the boundaries, and using a corollary of the Intermediate value theorem, you can conclude the existence of exactly one point of intersection on each segment, two in total.  
A: $\ln x$ does not exist for $x \le 0$ and $\frac 1{x-1}$ does not exist for $x = 1$.
So if there are solution they will exist on the intervals $(0,1)$ and $(1, \infty)$.
On these intervals the function $f(x) = \ln x - \frac 1{x-1}$ is continuous.
$f'(x) = \frac 1x - (-1)\frac 1{(x-1)^2} = \frac 1x + \frac 1{(x-1)^2} > 0$ so $f'(x)$ is increasing on the intervals $(0,1)$ and $(1,0)$. So if $f(x)$ ever "crosses the $x$-axis".  It can only do so at most once in each interval.
Taking the limits of $f(x)$ as $x \to 0^+; x \to 1^-; x\to 1^+$ and $x\to \infty$ we will see that $f(x)$ will have negative and positive values on each of these intervals.  So by the intermediate value theorem there will be values in these intervals where $f(x) = 0$.  
And as $f'(x) > 0$ there can only at most one on each interval, so there will be exactly one on each interval.
And, obviously, if $f(x) = 0$ then $\ln x = \frac 1{x-1}$.
