# Documentation

## Create a Rate Rule to Define a Differential Rate Equation

Many mathematical models in the literature are described with differential rate equations for the species. You could manually convert the equations to reactions, or you could enter the equations as rate rules. For example, you could enter the following differential rate equation for a species `C`:

as a rate rule in SimBiology®: ```C = vi - (vd*X*C)/(Kc + C) - kd*C```

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