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Microsoft Office Word 2007 includes builtin support for writing and editing equations. Previous versions used the Microsoft Equation 3.0 addin or the Math Type addin. Equation 3.0 was included in previous versions of Word and is available in Office Word 2007. Math Type was not included in previous versions of Word but was available for purchase.
If an equation was written in a previous version of Word and you want to edit the equation by using Office Word 2007, you need to use the addin that was used to write the equation. If you purchased Math Type, you need to have Math Type installed.
What do you want to do?
Change an equation that was written in Office Word 2007

Click the equation that you want to edit.

Make the changes that you want.
Change an equation that was written in a previous version
To change an equation that was written in a previous version by using the Equation 3.0 addin or the Math Type addin, you need to use the addin that was used to write the equation.
When you open a document that contains an equation that was written in a previous version of Word, you cannot use the builtin support for writing and changing equations unless you convert your document to Office Word 2007.
Change an equation that was written by using Equation 3.0

Doubleclick the equation that you want to edit.

Make the changes that you want.
To get help in Equation Editor, click Equation Editor Help Topics on the Help menu.
Change an equation that was written by using Math Type
To change an equation written with Math Type, you need to have Math Type installed.

Doubleclick the equation that you want to edit.

In Math Type, edit the equation.

On the File menu, click Close and Return to name of the document that contains the equation.
To get help in Math Type, click Contents and Index on the Help menu.
Convert your document to Office Word 2007
If you convert your document and save it as an Office Word 2007 file, you will not be able to use previous versions of Word to change any equations in the document.

Click the Microsoft Office Button , and then click Convert.

Click the Microsoft Office Button , and then click Save.
Note: After you convert your document, you need to save the document as an Office Word 2007 document, or the document will revert to the previous version.