二维随机变量（x,y）的概率密度为f(x,y)=Ae^-(x+2y),x>0,y>0,其他为0，求系数A以及x,y的边缘密度函数 设二维随机变量（X,Y)的联合概率密度为f(x,y)=Ae^...
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对x积分得y的边缘概率密度 f（y）=2e^-2y
对y积分得x 的边缘概率密度 f(x)=e^-x
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