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All versions of Microsoft Office Word produced since 2007 include builtin support for writing and editing equations. Versions prior to Microsoft Office Word 2007 used the Microsoft Equation 3.0 addin or the Math Type addin.
Note: The Equation command is unavailable in Compatibility Mode. To turn off Compatibility Mode, convert the document to the current Microsoft Office Word file format.
If an equation was written in a version of Word prior to Microsoft Office Word 2007 and you want to edit the equation by using the current version of Microsoft Word, you need to use the addin (such as Math Type) that was used to write the equation.
What do you want to do?
Write an equation
To write an equation, you can use Unicode character codes and Math AutoCorrect entries to replace text with symbols. For more information, see Insert a symbol or special character.
When you type an equation, Word automatically converts the equation into a professionally formatted equation.

On the Insert tab, in the Symbols group, click the arrow under Equation, and then click Insert New Equation.

Type an equation.
Insert a frequently used or preformatted equation

On the Insert tab, in the Symbols group, click the arrow next to Equations, and then click the equation that you want.
Add an equation to the list of frequently used equations

Select the equation that you want to add.

Under Equation Tools, on the Design tab, in the Tools group, click Equation, and then click Save Selection to Equation Gallery.

In the Create New Building Block dialog box, type a name for the equation.

In the Gallery list, click Equations.

Select any other options that you want.
Insert a commonly used math structure

On the Insert tab, in the Symbols group, click the arrow under Equation, and then click Insert New Equation.

Under Equation Tools, on the Design tab, in the Structures group, click the structure type that you want, such as a fraction or a radical, and then click the structure that you want.

If the structure contains placeholders, click in the placeholders and type the numbers or symbols that you want. Equation placeholders are small, dotted boxes in an equation .
Convert your preMicrosoft Office Word 2007 document to the current file format for Microsoft Office Word
If you convert a document created in a version of Microsoft Office Word prior to Microsoft Office Word 2007 and save it as a current Microsoft Office Word file, you will not be able to use versions of Word prior to Microsoft Office 2007 to change any equations in the document.

To use Microsoft Office Word 2007 to convert a document created in a version of Microsoft Office Word prior to Microsoft Office Word 2007, click the Microsoft Office Button , and then click Save As. Select Word document as your new file format and then click Save.

To use Microsoft Office Word 2010, 2013 and 2016 to convert a document created in a version of Microsoft Office Word prior to Microsoft Office Word 2007, click File, and then click Save As. Select Word document as your new file format and then click Save.