第1个回答 2012-06-08
(1)∫∫f(x,y)dxdy=1
c∫(0→1)x²dx∫(0→1)ydy=c·1/3·1/2=1 ∴c=6
(2)P{X<0.5,Y<0.5}=∫∫(D)6x²ydxdy
=∫(0→0.5)∫(0→0.5)6x²ydxdy
=6·x³/3|(0→0.5)·y²/2|(0→0.5)
=1/32
(3)fX(x)=∫(0→∞)f(x,y)dy
=∫(0→∞)6x²ydy
=6x²∫(0→1)ydy
=3x²
(4)EX=∫(-∞→+∞)xfX(x)dx
=∫(0→1)3x³dx
=3x⁴/4|(0→1)
=3/4
(5)cov(X,Y)=EXY-EXEY
EXY=∫(-∞→+∞)∫(-∞→+∞)xyf(x,y)dxdy
=∫(0→1)6x³dx∫(0→1)y²dy
=7/6
EY=∫(-∞→+∞)yfY(y)dy=∫(0→1)2y²dy=2/3
∴cov(X,Y)=EXY-EXEY=7/6-1/2=2/3