乙以4:1获胜的概率为
C(4,3)*(1/2)^4 *1/2=4/2^5=4/32=1/8也就是乙以4:2或4:3获胜的概率C(5,3)*(1/2)^5 *1/2+C(6,3)*(1/2)^6 *1/2=10/2^6+20/2^7=40/2^7=5/16比赛局数x概率p 4 (1/2)^4*2=1/8 5 C(4,3)*(1/2)^4 *1/2*2=1/8*2=1/4 6 C(5,3)*(1/2)^5 *1/2*2=102^6*2=5/16 7 C(6,3)*(1/2)^6 *1/2*2=20/2^7*2=5/16期望E=4*1/8+5*1/4+6*5/16+7*5/16=(8+20+30+35)/16=93/16=5.8125