The first question, is why use symboic evaluation in the first place? Mathcad is essentially a numeric calculator, with some symbolic capabilities tacked on. Generally, you should only use symblic evaluation when you have a definite and compelling reason to do so. Symbolic evaluation is not a replacement for, nor the equivalent of, numeric evaluation.
You can use the float keyword to control the number of places for symbolic evaluation. But note that float controls the wholw evaluation, not just the display. Using a small value of float will result in roundoff errors in the result (how much depending the the extent of the calculations), not just change the display value. You can also use the format results dialog, checking the option to apply it to symbolic results. That has its own quirks.
To be able to vary f you must make f define your results as functions with formal parameter f. You can then evaluate the function for any desired values of f.
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� � � � Tom Gutman