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

来源: 于 2010-04-26 15:02:38 [旧帖] [给我悄悄话] 本文已被阅读：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