将一枚硬币重复掷n次,以X和Y分别表示正面向上和反面向上の次数,则X和Yの相关系数等于（）
Hotmetal________asitgrowscooler.
Hefellasleepimmediatelylastnight;hemust________(be)verytired.
SinceWilliamhadbeenseriouslyillforseveralmonths,hisparentswereworriedabout________toreturntoschoolfull-time.
设f(x,y)在点(0,0)附近有定义,且,则
,则 D绕y轴旋转所成旋转体体积为_____
