Supposedly an easy way to remember equations in physics and maths, is only really useful at low levels.
It works like this - if I want to remember how to use the formula for density, Density = Mass / Volume , I'd put it into a triangle like so -
/ M \
/ D * V \
Now, if I want the formula for Density, I just cover up the D with my finger, and all that's left is the M/V bit. That's my equation. The same works for working out all the letters.
The idea is that you just remember the order of letters in the triangle rather than the equation. Not that much use, though.