The envelope command takes two arguments, the first argument is expression expr in the variables x, y and a parameter such as t, and the second argument is the name of the parameter.
envelope draws the envelope of the family of curves given by
expr=0 over the parameter t.
Input:
Output:
The envelope command can use variables besides x and y. In
this case, the second argument needs to be a list of the two variables
and the parameter.
Input:
Output: