I have to solve an equation which is $$\sum_{i=1}^N x_i = \sum_{i=1}^N y_i,$$ where

$$x_i = \frac{z_i}{1 + (K_i - 1) w}$$ and $$y_i = \frac{K_i z_i}{1 + (K_i - 1) w}.$$

The $z_i$ are all positive and add to $1$; the $K_i$ are positive but range from very small to very large (at least, one is strictly greater than $1$ and one is strictly smaller than $1$).

The solution ($w$) looked for is between two vertical asymptotes corresponding to $-1 / (K_{\text{max}} - 1)$ (which is negative) and $1 / (1 - K_{\text{min}})$ (which is greater than $1$). Considering the fact that, at solution, all $x_i$ and $y_i$ must be positive and smaller than $1$ (since they both must add to $1$), I have been able to find a left bound corresponding to the maximum value of $\dfrac{K_i z_i - 1}{K_i-1}$ (considering only the $K_i > 1$) and a right bound corresponding to the minimum value of $\dfrac{1 - z_i}{1 - K_i}$ (considering only the $K_i < 1$). These bounds are inside the asymptotes but I need a tighter interval.

I have not been able to go further. Any help will be more than welcome.