类似于全概率公式 P(X=x)=∑2=∑P(X=x,y=y,) =∑P(X=xY=y)PY=y) i=12.… P(Y=y)=∑=∑P(X=x,y=y =∑P(Y=yX=x)P(X=x) j=1,2类似于全概率公式 ( ) ( , ) 1 1    =  = = = = = = j i j j i ij P X x p P X x Y y ( ) ( ) 1 j j i j = P X = x Y = y P Y = y  = i =1,2,  ( ) ( , ) 1 1    =  = = = = = = i i j i j ij P Y y p P X x Y y ( ) ( ) 1 i i j i = P Y = y X = x P X = x  = j =1,2, 