This was a hard question. An argument with true premises and a false conclusion is certainly invalid, but invalid arguments don't always have true premises and a false conclusion.

For example, this argument is clearly invalid.

All men are mortal. Napoleon was mortal. Therefore, Napoleon was a man.

The premises are both true. The conclusion is not false. Yet the argument is invalid. The conclusion does not follow. Just because Napoleon was mortal that doesn't imply that he was a man. Cats, dogs and goldfish are mortal too.

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