数理

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1,E(x)=∫∫x(x+y)dxdy=∫dy∫x(x+y)dx（积分限都是0到1）=7/12,E(x^2)=∫∫x^2(x+y)dxdy=∫dy∫x^2(x+y)dx=5/12,所以D(x)=E(x