reply to u/phil98f at https://www.reddit.com/r/Zettelkasten/comments/10dbza7/equations_and_formulas_in_mathematics_physics_and/
Perhaps you've not gotten far enough in your studies to see the pattern yet, but most advanced mathematics texts (Hungerford's Algebra or Rudin's Real Analysis for example) and many physics texts are written as if they were pre-numbered zettelkasten. Generally every definition, theorem, corollary, proposition, and lemma in the text will be written out succinctly with its own unique number and arranged in some sort of branching order. Most of these texts you could generally cut up and paste onto cards and have something zettelkasten-like without any additional work.
I generally follow this same pattern and usually separate proofs on cards behind their associated theorems (typically only for those I've written out myself). Individual equations can be numbered, but I rarely give an equation its own card and instead reference them by card number with a particular equation line in parenthesis when necessary. I can then cross reference definitions, theorems, etc. easily as I continue building things up. Over time you can eventually cross reference various branches of math, physics, chemistry, biology, etc. For example I've got a dozen different proofs and uses of Schwartz' inequality in six different branches of mathematics which goes toward showing how closely knit various disparate branches of math can be.
In most cases I might also suggest against too heavy a focus on the equations, but on what they may say/mean, and what you can use it for. Alternately drawing diagrams or pictures of the relationships can be valuable. Understand it first then write it down. As an example you can look at Boyle's Law, Charles' law, Avogadro's Law and Gay-Lussac's law, but if you know and understand them properly then you should be able to write down and understand the more generic Ideal Gas Law, which shows how they're interlinked. Later in your studies you might also then be able to derive it from microscopic kinetic theory with statistical mechanics as well, then you'll be able to link up those concepts at that time.