Hello The question I have is how do mathematicians go about creating an equation to model a real system. I would like to know how they go about doing this, any links would be nice. To be more clear lets say scientists are trying to study how carbonic acid and temperature affect the pH of seawater this are two independent variables so some type of differential equation must be found to be able to model the system and see how each variable independently will affect the pH, this way by using partials derivatives they can find the max and mins of the function. I think that a hypothetical equation might be f(x,y)= x^2+y^3 where f(x,y) is the pH, x is the temperature and y is the carbonic acid. How do they get to the equation? That is a hypothetical example I came up with, to illustrate were i am coming from. Like I say I just want to know how scientist, mathematicians and engineers go about solving this real life problems to be able to model them. Thanks in advance P.S I have knowledge of multivariable calculus and of ordinary differential equations. I apologized for any spelling mistakes.