I realised I needed to show more information, which I now did:
f:X→XandA⊆X
Proof that: f(f−1(A))⊆A
This is my proof:
By defintion:
f−1(A)={x∈X∣f(x)∈A}
and
f(A)={f(x)∣x∈A}={y∈X∣∃x∈A:y=f(x)}⊆X
Therefore we can end the proof by a final definition:\
f(f−1(A))={y∈A:∃x∈f−1(A):y=f(x)}⊆A
Is this a legit "proof"? And is it even a proof, when i only use definitions?
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