how to know if an optimization problem has a closed form solution?

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I have this optimization problem which is linear and therefore convex. how should I calculate the closed form solution:

$\min_{\eta_i} \max_i \frac{\eta_i+\beta_i}{R_i}$ s.t. $\sum \eta_i=\alpha$ and $\eta_i \geq 0$

where $\beta_i$'s and $R_i$'s are all positive.

so I wrote it as the linear form of

$\min_{\eta_i} K$ s.t. $K\geq \frac{\eta_i+\beta_i}{R_i}$ and $\eta_i \geq 0$

which indicates a linear programming. How should I find the closed form solution.