-f(x) mean that functions in the x- axis are swapped from right to left on the graph.
A negative function is simply a function having its values all negative (i.e., f(x) < 0).
The domain rather known as the inputs of the function can be positive(+), but every output that is, the y-value must be less than zero.
A negative function has values that are negative for its domain.
It is important to note that;
Thus, -f(x) mean that functions in the x- axis are swapped from right to left on the graph.
Learn more about negative functions here:
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