# FindInstance with a symbolic constant

Please consider  FindInstance[m >= 0 && x >= 0 && x <= m, {x}]  for which, I would like a parametric answer x = m or x=m/2. But this yields an error:  FindInstance: The system contains a nonconstant expression m independent of variables {x}.  How can I achieve this goal?

• Perhaps Reduce is what you need?
– Johu
Sep 5 '18 at 13:42

FindInstance wants to have all the variables listed expicitly. So you can do:

FindInstance[m >= 0 && x >= 0 && x <= m, {x, m}]


FindInstance[m >= 0 && x >= 0 && x <= m, {x, m}, 5]


If you want a specific answer (like x==m/2), then you'll need to say that you want that form

FindInstance[m >= 0 && x >= 0 && x <= m && x == m/2, {x, m}, 5]


Thus FindInstance finds a numerical example, not a symbolic one. If you want to keep things symbolic, perhaps you can use Reduce or Solve, for example

Reduce[m >= 0 && x >= 0 && x <= m && x == m/2]


reduces to a smaller number of statements (two) that is equivalent to the four conditions input.