Basics of the Language

Commands in Mathematica are given in natural language form such as Solve or Simplify etc. The format of a command is the word starting with a capital letter and enclosing the argument in square brackets  

On the other hand, braces are distinct. They are used to designate lists or vectors as in  

When you want to clear the value of a given named variable there are three options  

The first two simply clear the current value while the last removes the name entirely. You need to remember this because if you start a session and assign a value to a variable, then that value will be retained in that variable until you either change or clear it. 

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