Let one source at $x\=0$ on a number line have strength 1; and the other at $x\=L$, have strength $k$.

Except for a proportionality constant, the combined strength at $x$ is given by

Figure 3.8.10(a) is a representative graph of $S\left(x\right)$ with $L\=10$ and $k\=5$.

Control-drag $S\left(x\right)\=\dots$
Context Panel: Assign Function

Write the equation $S\prime \left(x\right)\=0$ and press the Enter key.

Context Panel: Solve≻Obtain Solutions for≻$x$

Control-drag the first (and only real) solution.
Context Panel: Assign to a Name≻$\mathrm{Xmin}$ 

The graph of $\mathrm{Xmin}$ as a function of the relative strength $k$ in Figure 3.8.10(b) suggests computing

As the strength of the source on the right decreases, the minimum of $S$ moves toward $x\=1$.

As the strength of the source on the right increases, the minimum of $S$ moves toward $x\=0$.