从而 0 2 3 02160.4320.2880.064 0 3 X(X-2)~ 0.4320.5040.064 2 X(3-X)~ 0.2800.720从而 2 0 1 2 3 0.216 0.432 0.288 0.064 X        ( 2) 1 0 3 0.432 0.504 0.064 X X          (3 ) 0 2 0.280 0.720 X  X       