# Exercise 6: Plot a derived variable 
**(10 minutes)**

Revise AnalyzeVariables.C to make a histogram of the variable
**{ref}`pt <treeviewer-expressions>`**.  Don't forget to label the
axes; remember that the momenta are in *GeV*.

:::{note}
If you want to figure out what the bin limits of the histogram should
be, I'll permit you to "cheat" and use the following command
interactively:[^f85]

    tree1->Draw("sqrt(px*px + py*py)")
:::

[^f85]: If you compare this command with the computation of **`pt`** on the
    previous page, you may be either confused or irritated: When using
    C++ you have to access the n-tuple variables using pointer notation
    like "`(*px)`", while when using ROOT directly you can get away with
    just using the variable names like **`px`**. This is one of the
    reasons many folks prefer Python.