This depends greatly on what level you're looking at...
Algebra? Vector trig? Calculus? Diff.Eq?
IMHO, any instance where a theory is actually used
to describe a physical (real) situation (as an example)
that example is a "model" of reality.
You can't "prove" a model - you just try it,
then compare with the experiment.
If the model prediction is pretty close to the experiment,
you use that model again - if not, you toss it out.
If you're just starting, maybe most of the "equations"
are essentially definitions.
(Physics books tend to NOT distinguish equations with 3 lines).
Again, you can't prove a definition -
you keep useful ones and discard the non-useful ones.