Proof:A ∪ (B ∩ C) = (A ∪ B) ∩ (A ∪ C) bogus proof (by example)
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Sometimes students provide an example when asked for a proof. An especially common way of doing this is by providing a picture (such as a Venn diagram) in place of a proof. This is not sufficient because the proof only works for the specific example given; usually we ask you to prove things about a general situation.
Pictures are still extremely helpful when figuring things out and explaining them! They just usually aren't sufficient to proving a claim in full generality (unless that claim is that something exists, then a picture can be enough to describe the thing exists).