Where is Equation Editor?
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Microsoft Office Word 2007 includes builtin support for writing and changing equations. As a result, equation editor is not used, but instead equations are edited directly from within Word. To do this, click the Insert tab then click the Equation button. For information about writing equations in Office Word 2007, see Write or insert an equation.
If you wrote an equation using Microsoft Equation 3.0 in a previous version of Word, however, you still need to use Equation 3.0 to change that equation.
Change an equation that was created with 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.
Use the Office Word 2007 features
When you open a document that was written in a previous version of Word, you cannot use the enhanced equation editor functions for writing and changing equations unless you convert your document to Office Word 2007. To convert your document, do the following:

Click the Microsoft Office Button , and then click Convert.

Click the Microsoft Office Button , and then click Save.
View Demo
The following video shows you how to insert a predefined equation, create a new equation, modify an equation, and save the equation for later use.
