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Hi, I started reading this ![paper](https://github.com/Pythonsimulation/Blackholesimulationusingpython/blob/master/Black%20hole%20simulation.pdf) dealing with the simulation of a Schwarzschild black hole in python, what I didn't understand is how did the author of the paper derive ##v_1 (0) = \frac{1}{D \cdot \tan \alpha}## from ##v_0 (0) = \frac{1}{D}##?
Can anyone here kindly provide an answer.
(Below are screenshots from the paper)
[Mentors' note: edited to fix the Latex]