# 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.