Select the cells containing the formulas
Right-click and choose Format Cells
Go to the Protection tab
Check Locked
Click OK
Go to the Review tab
Click Protect Sheet
Enter a password if desired
Make sure Select locked cells is set as needed
Click OK
To hide formulas, select the formula cells
Right-click and choose Format Cells
Go to the Protection tab
Check Hidden
Click OK
Protect the sheet again to apply the hidden setting