Ms.Excel Tips Lock and Protect Cells Containing Formulas
You may want to let users change cells containg data without providing
them access to change formulas. You can keep cells containing formulas
under lock and key without having to protect your entire sheet or
workbook.
When we create a spreadsheet, most of us need to use formulas of some
sort. Sometimes, however, you might not want other users to tamper/
delete/overtype any formulas you included on your spreadsheet. The
easiest and most common way of barring people from playing with your
formulas is to protect your worksheet. However, protecting your
worksheet doesn't just prevent users from tampering with your
formulas, it also stops users from entering anything at all. Sometimes
you do not want to go this far.
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https://sites.google.com/site/excelknowitall/home/chapter-9