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x = R * cos(lat) * cos(lon)y = R * cos(lat) * sin(lon)

I'm not a very civil guy, so :What does the R represent ? at equator or at your point ?Are X and Y on the surface ? OR :

export you point in state planeimport it into QGIS using new layer from delimited text optionexport that layer in lat lon, or KML, or what ever...tell me what you have (post the point file), and tell me what you want...and I can transform and give you a tutorial for same

(ade_projsetsrc "LL84")(ade_projsetdest "SGC")(ade_projptforward pt)

Just adding an input.The coordinate conversion you are trying to.do is involving a math to unproject a projected point to a global coordnate system GCS (actual position in an ellipsoid/spheroid) which are measured in degrees.Assuming the xy coordinates given are Projected in UTM. Then you would need to tell which one of the 60 zones is the location of the coordinates. Each of the zones has the same easting and northing but different locations.You would also need to tell which datum you are converting to. This can be WGS84 or any other. There is a long list of datums. lat longs are not only WGS84.It looks like the equations you are trying are rotational transformations but not relating to geodetics.