When using the formula D=M/V, should the variables be capital or does it matter?

Answer 1

I don't think it technically matters, you just need to make sure you're not using a formula with a letter that could be mistaken for a number if it's lowercase, like "L". If you're writing it out, the way most people make a lower-case "l" is very similar to capital "I" or the number "1". The same problem could occur using a formula with the letter "b", because of its similarity to "6"

Answer 2

The V for volume should be capitalized, since lower-case v stands for velocity. Otherwise, it doesn't really matter. I prefer using a lower-case m for mass in general, and I only use M for specific objects, but that's just a personal preference. Likewise for d for density (actually I use the greek letter rho for density, but I'm guessing you're not teaching upper level physics!)

It's more important that you be consistent with yourself and with the textbook (if you use one). I find that the students interchange lower-case and upper-case letters in their own notes all the time regardless of what I do, anyway.