Using keyboard shortcuts
To open the Insert Function dialog box:
Select an empty cell and press Shift+F3.
To open a Function Arguments dialog box:
Select a cell containing a formula and press Shift+F3.
To insert a new Formula into a cell using the Function Arguments dialog box:
1. Select an empty cell, and then type the = sign.
2. Type the formula name and press Ctrl+A.
To insert a formula by typing it while being guided by the formula syntax tooltip:
1. Select an empty cell, and then type the = sign followed by the formula name and a ( sign.
2. Press Ctrl+Shift+A (in Excel version 2003 the syntax appears immediately after step 1 above).
Screenshot // Inserting/Editing Formulas