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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?

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    $\begingroup$ Perhaps Reduce is what you need? $\endgroup$ – Johu Sep 5 '18 at 13:42
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FindInstance wants to have all the variables listed expicitly. So you can do:

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

To see many answers:

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.

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