FFEccDYDX:

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   Compute the slope of an ellipse, dy/dx, at a particular true anomaly. 
   Here, x is along the major axis and y is along the minor axis.

   This function is called by the "FFEccTrueAnomZInc" function to find the true  
   anomaly at which the maximum cross-track separation occurs, assuming a pure 
   inclination difference (no right ascension difference). At this true anomaly, 
   the slope (dy/dx) of the ellipse is equal to the tangent of the argument of 
   perigee, w. 

   If the argument of perigee is passed as a third input, the 
   difference between the slope and tan(w) will be returned.

   Since version 7.
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   Form:
   dydx = FFEccDYDX( nu, e, w );
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   ------
   Inputs
   ------
   nu              (1,N)  True anomaly [rad]
   e                (1)   Eccentricity
   w                (1)   Argument of perigee (optional, default 0)

   -------
   Outputs
   -------
   dydx            (1,N)  Slope on the ellipse at each true anomaly

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	  Copyright 2004 Princeton Satellite Systems, Inc.
    All rights reserved.
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