Question about sets

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Originally Posted by Plato

Correct. So the OP
Is incorrect.

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OK, I see where I made a glitch. I was assuming that on a Venn diagram, the default way to represent A and B was ''partly separated'' and ''partly joined together'', but this isn't necessarily the case. When we only have a reunion and not an intersection, A and B don't have to be joined together at all, and the operation still works anyway.

Attachment 22984

In this picture, how can we be sure that all possible combinations of 4 sets A-B-C-D are represented ?

Quote:

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Originally Posted by Nombredor

OK, I see where I made a glitch. I was assuming that on a Venn diagram, the default way to represent A and B was ''partly separated'' and ''partly joined together'', but this isn't necessarily the case. When we only have a reunion and not an intersection, A and B don't have to be joined together at all, and the operation still works anyway.

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Venn diagrams are a wonderful way to see outcomes.
But they are a lousy way to prove anything.