second moment

二次矩，二阶矩

"Second, it came to China at the most opportune moment when China was in disunion at the end of the powerful and prosperous Han Dynasty. "
其次，它适值最佳的时机步人中土，其时中国正处在强大繁荣的汉代末期天下大乱的时期。收藏
The second unforgettable moment came five hours later at the pole vault pit
第二次令人难忘的时刻发生在五个小时之后的撑杆跳场地上。收藏

the expected value of the square of the deviations of a random variable from the point of origin

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