differential equations – Why does DSolve return the same DSolve expression?

I'm trying to find the closed solution for the classic chemostat model with a species and a resource (Monod answer) with the help of the DSolve () function. However, after running for 10 minutes, I find the same equation.

In:
Resolution[{ x'
R & # 39;
{X

10 minutes later

Outside:
Resolution[{Derivative[1][x]
Derivative[1][R]
{X

Screen capture