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Original Message (ID '126353') By jf:
In Response To 'Re: Re: a question about substitution' --------- Both solutions from Solve work in the equation you asked Solve to solve for. To select the preferred solution, you need to describe it in a way that Select or Cases can use. The difference in the internal structure of the two solutions is that the first is multiplied by -1. With Cases, we can describe a pattern that is -1 times something else. In[8]:= Cases[x /. {{x->-(Sqrt[-\[CapitalDelta]+\[Omega]]/(Sqrt[2] Sqrt[\[Omega]]))},{x->Sqrt[-\[CapitalDelta]+\[Omega]]/(Sqrt[2] Sqrt[\[Omega]])}}, - _ ] Out[8]= {-(Sqrt[-\[CapitalDelta]+\[Omega]]/(Sqrt[2] Sqrt[\[Omega]]))} See the attached notebook.