Well as the questions says...
An equation might be more meaningful in its math-notated form than it is in code. Being able to put math in comments could improve readabibity on my projects.
In .NET btw.
I just use multiple lines to do it thus:
// Work out average as: sum (values)
// --------------
// count (values)
//
// and distance between points as:
// _______________________
// / 2 2
// d = \/ (x1 - x0) + (y1 - y0)
//
// and the following function:
//
// 3 2
// f(x) = ax + bx + cx + d
No magic required for that at all. Don't you just love ASCII art?