Update randVar()

Redoing #43394

Ensure exercises use randVar() in math-format (is there a reason for it being in math-format rather than just math?) when generating variable names. Ensure variable names don't include u or v, which could be confused.

Part of me thinks students ought to be able to distinguish u and v, but then again, the exercises are meant to test their mathematical ability, not their ability to read the Latin alphabet.

Deals with #43236 and #43385.