TITLE: How to solve equations with logarithms, like this: $ ax + b\log(x) + c=0$
QUESTION [7 upvotes]: I encountered an equation of type $$ ax + b\log(x) + c=0$$ Here a, b, and c are constants. Does anyone know how to solve these type of equations? I guess this way:
$$\log(x)= \frac{c-ax}{b}$$
$$x= 10^{(c-ax)/b}$$
But I do not even know how to solve this too. Please help!

REPLY [4 votes]: Rewriting the equation gives
$$
ax+b\log(x)+c=0\\
ax+b\log\left(\frac abx\right)+c-b\log\left(\frac ab\right)=0\\
\frac abx+\log\left(\frac abx\right)+\frac cb-\log\left(\frac ab\right)=0\\
\color{#C00000}{(a/b)x}\,e^{\color{#C00000}{(a/b)x}}=(a/b)\,e^{-c/b}
$$
The Lambert-W function is the inverse of $xe^x$. Therefore,
$$
\color{#C00000}{(a/b)x}=\mathrm{W}\left((a/b)e^{-c/b}\right)
$$
and so
$$
x=(b/a)\mathrm{W}\left((a/b)e^{-c/b}\right)
$$