# solve() drops variables from equation - why?

this is my worksheet: https://cocalc.com/share/f7766c5e-2f4...

In line 47 I solve for d_k. However the result does not contain p_ges nor U. That does not make sense to me, and makes me suspicious towards the result. What could have caused that? This equation describes a physical phenomenon, and it would not make sense for d_k to be independent of U or p_ges.