I'm struggling to get what you mean by "distance"... Can you explain the problem in a bit more depth?

I am try to distance from attached formula.

I think I follow with what you're trying to achieve... Are you wanting to get the length of a line overlaid on a 3d surface?

If thats the case there is a much easier approach, assuming that you have a DEM of the area (this wouldve been used to derive the slope and aspect)

Use your DEM to create a surface, then using the SurfaceDraper, drape the line features over the surface so they have Z values, then use a length calculator with the dimensions set to 3 to calculate your length