Here is a sheet that should help and an animation demonstrating why values of x must be smaller than -2.383..
I still think that number should be -1-ln(4) and Mathcad allows for slightly bigger values only because of numerical inaccuracies in the solve block - anybody any comments on that?
A more detailled video (higher FRAME number) was uploaded here: http://communities.ptc.com/videos/4498
EDIT: Added 3D-plot
