f(x,y)=(1/2)(x+y)e^(-(x+y) f(x) =∫[0到∞](1/2)(x+y)e^(-(x+y))dy = (1/2)∫[0到∞]xe^(-(x+y))dy + (1/2)∫[0到∞]e^(-(x+y))dy = (1/2)xe^(-x)∫[0到∞]e^(-y)dy + (1/2)e^(-x)∫[0到∞]ye^(-y)dy = (1/2)xe^(-x)+ (1/2)e^(-x) = (1/2)(1+x)e^(-x), x.
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