<p>若问道题，<br/>我的疑问是y的密度函数似乎用x1与x2的卷积求是不对的。</p><img src="file:///C:/Documents%20and%20Settings/Administrator/桌面/TCQ2X3)%25%7D3Z07PNO@DRUCR8.jpg" border="0" alt=""/>