no defined variables and redefing them?

Hello,
i am currently making a calculation with Mathcad Prime 9.0, where i got 3 different load cases and i want them all in 1  sheet.

i made a few formulas at the beginning, and will later calculate my loadcases with them. For that i wanted some variables to be empty, so that they will just shown as their variable name, so i can later change this variable in every load case to a different number.

1. When i just use a non defined variable, the whole formula cant used further
2. When i give the variable a value and change it later, it will not recalculate the formula with the updated variable.

Is there a Solution for this? Or is this just how Mathcad works?