For my midterms and finals, I'd give each student a 3x5 notecard, which they could use on one side to write anything they wanted. In the real world, anyone can look up things like equations, so I figured it was fair. After all, I wanted to make sure they knew the operations. Incidentally, every year, the vast majority of students would say, 'After I copied all the notes on the card, I could remember them and didn't even need it!' Until one year a student came in with his card having the tiniest handwriting I ever saw. I couldn't read it. Turns out he was wearing both contacts AND his glasses, giving a major magnification effect....or so he told me years later.