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I suggest to add the function ctrl+enter to split an expression in Prime 3.1 like it is the case in Mathcad 15.
You're in luck! PTC has already indicated that they intend to include this in Prime 4.0.
Sorry!
We can do it in Prime 3 too
See the picture and the attach (Prime 3.1)
I only have Prime 1.0 on my current computer, but I was able to view the XML file for your worksheet and see your trickery.
P.S. I was also going to point out your error, but I see you have already fixed it yourself.
I replace a big symbolic result on a picture by the preparing a Mathcad sheet for publication.
See please one example here (the Fig. 7.2)