How did you learn to write? You practiced until you got it right. It is the same with formulas only that it is not something that you will use to communicate like a language. A formula is a mathematical language that is communication between two variables. Hypothetically let us say those two variables are V and M, where V stands for volume and M stands for mass. In this case, the relationship between volume and mass is to be derived. This can be done by taking into consideration, what happens when? Does the volume increase with increase in mass in which case, both variables are directly proportional. OR does the volume decrease with increase in mass and in that case, both variables are indirectly proportional.
Understanding the entire formula will require conceptual understanding. Hence, it could be trigonometry formulas’, statistics formulas or calculus formulas – You need to be very careful about understanding the concept well. Once the concept is understood then remembering a formula is not a big deal.
Yes, we know it is not theory to memorize or understand but then there is a theory hidden behind the formula. All you need to do is understand the theory and then getting the variables and establishing a relationship between them is easy. For instance, you may add, subtract, divide or multiply depending on the conceptual understanding but unless you know what is what, you don’t know what to subtract from what and what the necessary steps are.