To demonstrate how to use equations when modeling parts, we will create the following simple rectangular cuboid with the variables shown in the screenshot:

To start, we will define our global variables. To do this, follow these steps: 

1. Open the equations manager by right-clicking on the part name on the top of the design tree, then choose Hidden Tree Items | Equations | Manage Equations..., as highlighted in the screenshot:

2. Input the W and H variables under Global Variables and input the values as highlighted in the screenshot. Click OK after defining the variables:

3. Verify that the variables are already defined by checking the new folder in the design tree under the name Equations, which will show all of our variables. The following screenshot highlights the new Equations folder:

4. Start creating our rectangular cuboid by inputting the variables instead of the lengths. When dimensioning the width, we can input ="W" instead of the number 5, as shown in the screenshot. Note that the dimension will then show as ∑ 5 to indicate the involvement of the function of the equation:

5. Dimension the depth by inputting ="W"*2, as shown in the screenshot. Similar to the width, the depth dimension will be displayed as ∑ 10 to indicate the involvement of the equations function:

6. Apply the extruded boss feature; we can follow the same technique. When specifying the dimension, we can input ="H", which will result in the value 8, as highlighted in the screenshot:

After applying the extruded boss feature, we will have the complete rectangular cuboid, as shown in the following screenshot:

Note that we can define our variables using any term we want. It can be a single letter or a word. A good practice is to use a term that would make it easier for us to recall while creating the model. 

Now that we know how to apply equations, let's examine how easy this will make implementing modifications.