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Determine whether [itex]f: \mathbb{Q} \times \mathbb{Q} \longrightarrow \mathbb{Q} [/itex]

[itex] f(a/b, c/d) = (a+c)/(b+d) [/itex]

is a function.

Just some questions. The book's hint is that it is not a function, so what are some techniques for showing something is not a function? Also, how would I go about the whole f(a/b, c/d) part? Thanks.