you should derive math equations yourself, from scratch, even if you already know them by heart. you might not always be able to look them up, and even if you can there is knowledge and understanding to be gained in the process. plus it helps you practice doing derivations and troubleshooting mistakes, since you already know the answer. ive often found equations that are commonly used in my work are based on assumptions that no one will ever tell you about, which is dangerous because people will blindly use them in situations that violate those assumptions. this matters a lot to me because im an engineer, but its true for pretty much everyone.

also computer algebra programs are no substitute for doing the algebra yourself.