I am reading a lesson about mass transfer and diffusion and I have troubles to find the same solution given in the book. 

> 
> The differential equation is written as :
>  $$\frac{\mathrm{d}\left(\frac{-cD\_{12}\frac{\mathrm{d}z\_1}{\mathrm{d}z}}{1-\frac{z\_1}{2}}\right)}{\mathrm{d}z}=0$$
> 
> 
> 

My reasonning :

$$\begin{align}\frac{\mathrm{d}\left(\frac{-cD\_{12}\frac{\mathrm{d}z\_1}{\mathrm{d}z}}{1-\frac{z\_1}{2}}\right)}{\mathrm{d}z}=0 &\Longleftrightarrow \frac{\mathrm{d}}{\mathrm{d}z}\left(\frac{\mathrm{d}z\_1}{(1-z\_1/2)\mathrm{d}z}\right)=0\\ &\Longleftrightarrow \frac{\mathrm{d}z\_1}{(1-z\_1/2)}=K\_1 \mathrm{d}z \\ &\Longleftrightarrow -2\log(z\_1-2)=K\_1z+K\_2\end{align}$$

But I don't understand how to find $Ln(1-z\_1/2)$..where I guess $Ln$ means $\log$. $z\_1$ is a function of $z$. I know that $\log(1/x)=-\log(x)$ but ... 

Can someone help me please? Thank you in advance