To Create an Equation to Define the Height Variable

In the Feature Modeling Global Variables dialog, click the Equations tab.
Click the New icon.
A new row is inserted in the Equation list box, and the equation editor opens (if not, double-click the empty row to open the editor). Below the input field, the editor displays a list of Available Variables that may be used in creating the equation.
In the input field, type Height=0.5*(.
In the Available Variables list, double-click both Length and Width.
They both are inserted into the equation, with a + sign between them.
In the equation input field, complete the equation with a closing bracket ).

Creating the equation
Click the highlighted row in the Equations list box to add the equation (and close the equation editor).
Open the Variables tab and notice that the new variable Height has been added.