The document discusses how air traffic control (ATC) calculates the minimum safe distance between two aircraft that are crossing paths at an angle. It explains that ATC uses Pythagorean theorem and calculus, setting the derivative of the distance function equal to zero, to find the point of minimum distance. The solution is that the minimum distance occurs when the two aircraft pass each other at the midpoint between their positions. Some key equations and the final solution of minimum separation distance are also presented.