Guity
In Excel 2010
Goto Insert Equation, A box will appear on the screen and an Equation Toolbar appears at the top of the screen, Select the various equation parts and type numbers etc as required.
In Excel 2007,
On the Insert tab, in the Text group, click Object.
In the Object dialog box, click the Create New tab.
In the Object type box, click Microsoft Equation 3.0
In the Object dialog box, click OK.
Use the symbols, templates, or frameworks on the Equation toolbar to edit the equation.
Refer: http://office.microsoft.com/en-us/excel-help/use-equation-editor-in-excel-HP010215799.aspx
In Excel 2003,
Goto the Insert menu, click Object, and then click the Create New tab.
In the Object type box, click Microsoft Equation 3.0.
Build the equation by selecting symbols from the Equation toolbar (toolbar: A bar with buttons and options that you use to carry out commands.
To display a toolbar, press ALT and then SHIFT+F10.) and by typing variables and numbers. From the top row of the Equation toolbar, you can choose from more than 150 mathematical symbols. From the bottom row, you can choose from a variety of templates or frameworks that contain symbols such as fractions, integrals, and summations.
refer: http://office.microsoft.com/en-us/excel-help/insert-an-equation-with-equation-editor-HP001118120.aspx