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Sometimes, people tend to use metaphors from arithmetics to refer to something in ordinary life. For example, to mention some claims or preferences common among people, the expression 最大公約数 'greatest common measure' is used even though it is not about numbers. I kind of understand that because there is no immediate counterpart that expresses the corresponding concept for non-numbers. But I don't understand why people say 未知数 'unknown variable' for some non-number that is unknown. There is a word 未知 'unknown' that is related to and is simpler. Why do people say things like これからの情勢は未知数だ instead of saying これからの情勢は未知だ?

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I'm rather tempted to say something like "idiomatic usage; look no further into it". –  Flaw Dec 20 '11 at 23:28
    
We have the same usage in Chinese. –  fefe Dec 21 '11 at 0:33
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@Flaw: It is indeed an idiomatic usage, but I think that the point of the question is what the nuance of これからの情勢は未知数だ is compared to これからの情勢は未知だ (or any other similar usages of 未知数 vs 未知). I feel that there is certainly some difference between them, and this difference should be the reason why some people choose to use the former. But I cannot get a grip on what the difference exactly is. –  Tsuyoshi Ito Dec 21 '11 at 1:00
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In English we have the idiom "lowest common denominator", which is fairly unrelated to the mathematical meaning... we also say "unknown quantity" in some situations where "unknown" would suffice. –  Karl Knechtel Dec 21 '11 at 2:01
    
It seems that the question tag is the answer to the question. –  Flaw Dec 22 '11 at 9:30

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up vote 7 down vote accepted

This is not necessarily an idiomatic structure. English uses the exact phrase when talking about unknowns.

"Candidate X is an unknown variable so anything can happen once he/she enters the race."

Mathematical terms tend to be constants (yet another mathematical term) and many are actually co-opted from real world use to talk about mathematics. Alphabets are similarly used such as "crossing your t's and dotting your i's" to indicate finishing up properly, and "teaching him all the steps from A to Z", or referring to God as "the Alpha and the Omega" to indicate all encompassing. In Japanese, such things are "ア から ン".

The point is that, these terms are rather similes rather than idiomatic. They tend to be directly comparative rather than obliquely obscure.

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+1 for "similes rather than idiomatic". –  Dave M G Dec 21 '11 at 8:53
    
I can't make sense of から in the context "ア から ン"; I thought it implied some form of causality. –  Karl Knechtel Dec 21 '11 at 20:54

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