Select the cells that contain the formulas that you wish to hide.
If you want to hide every formula, select the entire sheet by clicking in
the square to the left of the Column labels, above the Row labels,
Then, <Format> <Cells> <Protection> tab,
Click on "Hidden", then <OK>.
Now, you have to protect the sheet in order for this to take affect.
If this protection inhibits some of the functionality of the sheet, you can
unlock any affected cells beforehand, even the ones that contain the hidden
formulas (means users can replace hidden formula with data entry).