How to hide formula in an excel file

Format the cells and on Protection tab check "Hidden", if you subsequently protect the worksheet those cells with Hidden status will no longer display formulae.

Not really, your only other option would be to hide the formula bar but that's ill advised and easy to reverse... ie it is rarely a good idea to alter the User Interface of another user without their prior consent....

(You don't need to Lock the cells when you protect the sheet for the formula to remain hidden but obviously the end user could just end up typing over your formula given they are not aware there is a formula present in the cell)

Of what exactly ? I'm presuming you mean the Unlocked / Hidden approach...

Basic Example... do the following in a new worksheet

A1: 10

B1: =A1

With B1 selected press ALT + O -> E -> Protection Tab -> uncheck Locked -> check Hidden -> OK

Now ALT + T -> P -> P -> enter Password: test -> reenter password to confirm.

You should now find you can no longer see the formula in B1 but you can still override it, ie you can enter anything you want to into that cell.

You will find however that the other cells are all locked ... so if this is an approach you want to put into action, prior to protecting the sheet unlock all cells you want to be editable the default status of all cells is to be locked upon protection... remember you can format cells simultaneously by selecting the appropriate cells prior to bringing up the format window, subsequent formatting choices are applied to the entire selection of cells.