Author 
Comment/Response 
Bill Simpson

01/23/13 12:21pm
Eliminate those "_", those mean something very different from what you think and cannot be used the way you are using them.
Use ";" between statements in a single cell to avoid even more confusing problems in the future. Sometimes you can get by without doing that, but it is a bad practice.
In[1]:= Atwo=5;Aone=5;Athree=5; Bone=5;Btwo=5;Bthree=5; Cone=5;Ctwo=5;Cthree=5; Done=5;Dtwo=5;Dthree=5;
Change the syntax you use for Solve.
Solve[k== (Atwo Cthree Done+Athree Ctwo Done+Atwo Cone DthreeAone Ctwo DthreeAthree Cone Dtwo+Aone Cthree Dtwo)/(Btwo Cthree DoneBthree Ctwo DoneBtwo Cone Dthree+Bone Ctwo Dthree+Bthree Cone DtwoBone Cthree Dtwo),k]
From In[1]:= Power::infy: Infinite expression 1/0 encountered.
Out[2]= {}
Which is because your denominator is zero.
In[3]:= (Btwo Cthree DoneBthree Ctwo DoneBtwo Cone Dthree+Bone Ctwo Dthree+Bthree Cone DtwoBone Cthree Dtwo)
Out[3]= 0
Add 1 to your numerator and denominator just to show that it works
In[4]:= Solve[k== (Atwo Cthree Done+Athree Ctwo Done+Atwo Cone DthreeAone Ctwo DthreeAthree Cone Dtwo+Aone Cthree Dtwo+1)/(Btwo Cthree DoneBthree Ctwo DoneBtwo Cone Dthree+Bone Ctwo Dthree+Bthree Cone DtwoBone Cthree Dtwo+1),k]
Out[4]=
{{k> 1}}
Or in this case your expression is simple enough that you don't even need to use Solve.
In[5]:= (Atwo Cthree Done+Athree Ctwo Done+Atwo Cone DthreeAone Ctwo DthreeAthree Cone Dtwo+Aone Cthree Dtwo+1)/(Btwo Cthree DoneBthree Ctwo DoneBtwo Cone Dthree+Bone Ctwo Dthree+Bthree Cone DtwoBone Cthree Dtwo+1)
Out[5]= 1
Note: Attaching small simplified notebooks, rather than screenshot images that just require someone else to type it all back in again to verify the answer is correct and really works, is almost always better.
URL: , 
