[理工] [機率]-期望值與變異數
請問倒數第三行如何發展成倒數第二行
http://i215.photobucket.com/albums/cc91/serflygod/0424.jpg
完全不知道怎麼算
我觸礁了..OTZ
懇請善心人士指點
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5年前
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