Question

If f(x) is a function and f(1) = 5, then which of the following could not be true?

f(1) = 1
f(2) = 1
f(5) = 5

Answer 1

If f(x) is a function, there is only one output possible for each input. In terms of your question, there can only be one value for f(1).

Basically f(1) can't be anything except 5.