Memorize equations- you'll thank me later. It's just so much easier to just know it rather than try and reason it out via the passage. However, you can reason out a lot of equations if you understand the underlying relationships. But generally, if there's an equation in your prep book you should study it. Any equation that isn't taught in a standard prep book (EK, Kaplan, TBR, even TPR) will be given in the passage.