If you have a workbook with lots of formulas and you want to protect those formulas from being amended by other people who share your workbook, then you can!
You need to follow these steps:
1. Press the Go To Special shortcut CTRL+G
2. Select the Constants box and press OK (this highlights all the non-formula cells)
3. Press CTRL+1 to bring up the Format Cells dialogue box
4. Select the Protection tab and Un-check the Locked box
5. In the menu ribbon go to Review > Protect Sheet > then enter your custom password (optional)
This will lock all the cells that are not constant, so all the formula cells 🙂