What is "someone you can't live without"?

來源: 於 2010-04-26 15:02:38 [舊帖] [給我悄悄話] 本文已被閱讀：次

如何定量描述？For example, there are two real roots when b*b-4*a*c>0, while there are two complex roots when b*b-4*a*c