# Exercise 7: Trig functions 
**(15 minutes)**

The quantity theta, or the angle that the beam makes with the *z*-axis,
is calculated by:

$$
\theta = \arctan\left(\frac{p_{T}}{p_{z}}\right)
$$

The units are radians. Revise your script to include a histogram of
theta.

:::{note}
I'll make your life a little easier: the math function you want is
`ROOT.TMath.ATan2(y,x)`, which computes the arctangent of y/x. It's
better to use this function than `ROOT.TMath.ATan(y/x)`, because the
`ATan2` function correctly handles the case when *x*=0.
:::

:::{figure-md} too_old_for_this-fig
:class: align-center

<img src="too_old_for_this.png" alt="xkcd too_old_for_this" width="80%">

<https://xkcd.com/447/> by Randall Munroe
:::