Equations aren’t supported in Excel Online. Because Equation Editor isn’t available in Excel Online, you won’t be able to insert them. When you open a workbook that has equations, they won’t be shown.
If you have the Excel desktop application, you can use the Open in Excel button to open your workbook to view and insert equations there. Here’s how:
Click Open in Excel and view or insert an equation.
For news about the latest Excel Online updates, visit the Microsoft Excel blog.