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Is there an "udecided"undecided" assertion of which a proof that it's not undecidable is known?

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Just a curiosity:

Is there an assertion of which a proof (formalizable, say, in ZFC) is not known but a proof that it's not undecidable (in ZFC) is known?

Edit: after the comments, I think the actual question was

Is there an ("interesting") assertion of which neither a proof (formalizable, say, in ZFC) of it or its negation is known but a proof that it's not undecidable (in ZFC) is known?

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Is there an "udecided" assertion of which aproofisnotknownbut a proof that it's not undecidable is known?

Just a curiosity:

Is there an assertion of which a proof (formalizable, say, in ZFC) is not known but a proof that it's not undecidable (in ZFC) is known?

Edit: after the comments, I think the actual question was

Is there an assertion of which neither a proof (formalizable, say, in ZFC) of it or its negation is known but a proof that it's not undecidable (in ZFC) is known?

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