How can i make equation in Solidworks 2013?

Hello raoufbelal,
I have made this tutorial to explain you how to use equations.
Remember the use of equations could be trick, specially at CSWP exam, link the values of equations is simple, but build the model in a way it rebuild as intended as you change the values, is another history.
Anyway, I hope this little tutorial helps you understand how to use it, and sure to help you out at your certification test.
All the best.

1. Step 1:

I will be making a cube as example for this tutorial, but once you learn the use you can apply the knowledge in any of your designs.
By default, Equations are not visible at Design Tree, we must start our tutorial by activating it.
First of all, open your SolidWorks and before making any new part, go to Menu and choose Options.

2. Step 2:

This will open SolidWorks System Options.
Once there at left list choose FeartureManager.
At right place you can see Equations, click the drop down menu and change it from Automatic to Show.

3. Step 3:

Now start a new model.
As you can see at your Design Tree, Equations is visible now.

4. Step 4:

Before start to model, right click Equations icon and choose Manage Equations.

5. Step 5:

The following pop up window you show, and now we will start to insert the values.

6. Step 6:

First click on the blank line right below Global Variables and type A.

7. Step 7:

Next, click on Value / Equation and enter the value 100. Once done press enter.

8. Step 8:

As you can see now, our Global Variable "A", Evaluates to 100 and we have a new blank line to make new Variables.

9. Step 9:

Now lets make one B variable. As you can see you can make use of arithmetical signs to get any results you want.

10. Step 10:

For the C variable, I will be using another variable to get a value I want.
After you type the name of the variable, click on Value and choose the Global Variable A.

11. Step 11:

Now to complete our set up make it divided by 2. You should have the same results as the picture.
If it is right, click OK to save your set up.

12. Step 12:

For next step, create a sketch and make a rectangle (in any plane you want).

13. Step 13:

Now time to link the dimensions to the values you previously set up.
Use Smart Dimension as usual, but instead of the values type the signal "=" (equal), doing so, like the picture, the pop up to link the value showing the Variables you already made will show.

14. Step 14:

Click on the option A (100).

15. Step 15:

The result should be this.

16. Step 16:

Press enter and you will see it changes the A for 100, but not like usual it has a Globe icon on its left.

17. Step 17:

Press enter (or click on the green check) to close the dimension edit, and you will see the result. Note that the dimension has the Equation Symbol before the Value, it means that your dimension is linked.

18. Step 18:

Now lets do the same with the other dimension of our rectangle, linking it to the variable B. The result should be like the picture.

19. Step 19:

Now lets close our Sketch and extrude it. As in the sketch, instead of value, let type the equal signal (=) to be able to link its value.

20. Step 20:

Once again you will notice the globe icon at the left side of the value.
Click Ok to finish the operation.

22. Step 22:

To change the values you must right click Equations on your Design Tree and choose Manage Equations, just like you did to first set up it.

23. Step 23:

Now we can change the values to see if it works as we want to, change the values as the picture.

24. Step 24:

Click OK and rebuild your model.

25. Step 25:

Note that if the values you change at equations makes the model geometrically impossible to build, you will get build error message, also if you are building a model with fixed and linked (equations) dimension, you must pay attention how to build, how to link the values, especially at CSWP Exam, instead of help it could mess all your model and make you lose a precious time trying to figure out what is wrong with the model. But if you do good, it saves you a lot of time.