概率方差D(X)公式简介：Sigh, sigh, the world is difficult and people are confused, but what can we do? Dynasties changed, and the bright moon at that time changed its direction. Jiang Sheng hummed a song and headed to the next city.
Sigh, sigh, the world is difficult and people are confused, but what can we do? Dynasties changed, and the bright moon at that time changed its direction. Jiang Sheng hummed a song and headed to the next city.
