Do they both mean "is it not/isn't it"?
e.g. Do この車は新しくないんですか and この車は新しいんじゃないですか both mean ”Isn’t this car new?”？
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Both can be a negative rhetorical question, but are used in different situations.
この車は新しくないんですか is "Isn't this car new?" It sounds like they are talking about the age of one specific car. More literally, "As for this car, isn't it new?"
この車は新しいんじゃないですか is "Isn't this one a new car?" It sounds like they are finding a new car among many cars. More literally, "This car is new, isn't that right?"