The core issue is you have used sqrt for square root, when it should be Sqrt. All of Mathematica's reserved symbols begin with upper case. Therefore, it is also good practice to begin user symbols with lower case to avoid conflict. Also, with regard to the formatting in your notebook, all of the expressions are entered in a single cell. If you enter these expressions one at a time, each in their own cell, and evaluate them each in turn as you go, you will be able to confirm immediately that the calculations are developing as you expect. Later it is easy to put semicolons at the end of expressions when you don't want to see the output in an output cell, and you can evaluate the entire sequence by selecting Evaluate Notebook from the top menu bar.
Best regards,
David
Notebook attached
Attachments: