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Use plot(y, (x, 5, 20)) — use y instead of y(x). When you pass y(x) as an argument, where y is a Python function, it gets evaluated right away: it doesn't plug in different values for x, it just tries to evaluate y(x) as a number. That's where the error messages come from. If you plug in y instead, it tests to see if it is a function and if so, it plugs in x-values in the given range.
