We notice that the value of E at r naught is negative, as it should be. It's a negative number.

我们主要到在r圈时E的值为负，和它本来的值一致，是一个负数。

麻省理工公开课 - 固态化学导论课程节选

The basic definition-- the expected value of some random variable x--E--I guess I should have said that a random variable is a quantity that takes on value.

最基本的定义,某一个随机变量X的期望值E,我应该提到过,随机变量是一个可以取值的数

耶鲁公开课 - 金融市场课程节选