I know after i ll explain how to hide a formula, no formula will be hidden from who so ever reads it...
but still, here the trick goes...
Write the formula as usual.
Right Click on the cell/s and Select "Format Cells..." (or press Ctrl+1)
Select "Custom" (under "Category:") at left pane
Under "Type:" at right side, Type one Space
Press Enter
Woop, its done
BUTTT
If some one searches anything by formula under search options, then SORRY, no escape...
Said
Its a good way of hiding formulas, as while preparing presentaions...some cells are not important to be shown which contains formulas.. so it is helpful....