Question: Expressions to large to solve??

Hi, I'm hoping someone in here will be able to help me.

I've made two files in which I need to find a symbolic expression for the difference between two mathematical models of a mass, damper, spring system:

View 8371_ID3dof.mw on MapleNet or Download 8371_ID3dof.mw
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and
View 8371_ID4dof.mw on MapleNet or Download 8371_ID4dof.mw
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The first file , ID3dof.mw, works fine. The second file, ID4dof.mw, is made in the same way as the first. The only difference between the two, are the dimensions of the matrices. However, where ID3dof finds the result in a second, ID4dof is unable to finish evaluating and Maple uses more and more system resources.

I've narrowed the problem down to the calculation of the matrix called "Du" and in particular the command MatrixInverse(G11-Pt), "G11" and "Pt" are both 3x3 and full rank. The elements of "G11" and "Pt" are quite long equations and I believe this is whats causing the problem, but I haven't been able to make them any shorter.

I have done the same calculations numerically and  everything works out fine, but I would really like to have the symbolic expression, since it's easier to relate to the physical system.

If someone could suggest how to optimize my code or in some way do a work-around of the problem, I would be very grateful...

Kind regards

Jakob