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å¨Aè¢ååå«åå°1ç½1é»ï¼2ç½ï¼2é»æ¦ç为
P(1ç½1é»)=C3(1)*C2(1)/C5(2)=3/5
P(2ç½)=C3(2)/C5(2)=3/10
P(2é»)=C2(2)/C5(2)=1/10
å¨Bè¢ååå«åå°1ç½1é»ï¼2ç½ï¼2é»æ¦ç为
P(1ç½1é»)=C5(1)*C3(1)/C8(2)=15/28
P(2ç½)=C5(2)/C8(2)=5/14
P(2é»)=C3(2)/C8(2)=3/28
é»çä¸ç½çæ°å·®çç»å¯¹å¼ä¸ºXå¯åï¼0ï¼2ï¼4
P(x=0)=3/5*15/28+3/10*3/28+1/10*5/14=109/208
P(x=2)=138/280
P(x=4)=33/280
Xçæ¦çåå¸
X 0 2 4
P 109/280 138/280 33/280
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EX= 0X109/280+2X138/280+4X33/280=51/35
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