All cells are locked, ie protected if you protect the worksheet by default.
That is is to say, if you protect the worksheet, with or without a password,
all cells will be locked and a user cannot change them.
To allow changes in a protected worksheet, right click the cells you require
Protection Tab and Uncheck 'Locked' then protect the sheet.
To protect only cells that contain formulas, highlight the whole sheet or
the area that concerns you, right click > Protection and uncheck 'Locked'
Go Edit > Goto > Special > Check 'Formulas' This will highlight all cells
that contain formulas. Right click one of them, Format Cells > Protection >
Check 'Locked' > OK
Regards,