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Let A be a finite set. If f: A->A is an ...

Let `A` be a finite set. If `f: A->A` is an onto function, show that `f` is one-one also.

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`A` be a finite set
`f:A→ A` is onto that means every element in `Y` set has a preimage in `X` set
That also means that every element in `X` must have only one image in `Y` because both `X`,`Y` are equal so the function is one - one.
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