由(1)知,f(x)在(x1,+∞)上单调递减,且x2>x1,∴2x1>x2.
19.解 (1)设Xk表示打完k个球后甲的得分,则此时乙的得分为Yk=k-Xk.
显然Xk~B(k,p).若甲的得分比乙的得分至少多2分,则Xk-(k-Xk)≥2,即Xk≥+1.
当k=3时,p3=P(X3=3)=p3(1-p)3-3=p3.
当k=4时,p4=P(X4=3)+P(X4=4)=p3(1-p)4-3+p4(1-p)4-4=4p3(1-p)+p4=p3(4-3p).
(2)由(1)得q3=q3,q4=q3(4-3q).
p4-p3=p3(4-3p)-p3=3p3(1-p).
q4-q3=q3(4-3q)-q3=3q3(1-q).
由=4,得=4,
又p+q=1,则=4,解得p=.
(3)当k是奇数时,
pk=PXk=+PXk=+PXk=+…+PXk=;
当k是偶数时,
pk=PXk=+PXk=+PXk=+…+PXk=.
则p2m+1=P(X2m+1=m+2)+P(X2m+1=m+3)+P(X2m+1=m+4)+…+P(X2m+1=2m+1),p2m=P(X2m=m+1)+P(X2m=m+2)+P(X2m=m+3)+…+P(X2m=2m),p2m+2=P(X2m+2=m+2)+P(X2m+2=m+3)+P(X2m+2=m+4)+…+P(X2m+2=2m+2).
p2m+1=pm+2qm-1+pm+3qm-2+pm+4·qm-3+…+p2mq1+p2m+1q0=()·pm+2qm-1+()pm+3qm-2+()·pm+4qm-3+…+()p2mq1+p2m+1q0=pm+2qm-1+pm+3qm-2+pm+4qm-3+…+p2mq1+pm+2qm-1+pm+3qm-2+pm+4·qm-3+…+p2mq1+p2m+1q0=q(pm+2qm-2+pm+3qm-3+pm+4qm-4+…+p2mq0)+p(pm+1·qm-1+pm+2qm-2+pm+3qm-3+…+p2m-1q1+p2mq0)=q·p2m-qpm+1qm-1+p·p2m=p2m-pm+1qm,即p2m+1-p2m=-pm+1qm.
同理q2m+1-q2m=-qm+1pm,
则p2m+1-p2m-(q2m+1-q2m)=qm+1pm-pm+1·qm=pmqm(q-p),因为p+q=1,且p2m+2=pm+2qm+pm+3qm-1+pm+4qm-2+…+p2m+1q1+p2m+2q0=()pm+2qm+()pm+3qm-1+()pm+4qm-2+…+()p2m+1q1+p2m+2q0=pm+2qm+pm+3qm-1+pm+4qm-2+…+p2m+1q1+pm+2qm+pm+3qm-1+pm+4qm-2+…+p2m+1q1+p2m+2q0=q(pm+2qm-1+pm+3qm-2+pm+4qm-3+…+p2m+1q0)+p(pm+1qm+pm+2qm-1+pm+3qm-2+…+p2mq1+p2m+1q0)=qp2m+1+p(pm+1qm+p2m+1)=p2m+1+pm+2qm.
又p2m+1-p2m=-pm+1qm,所以p2m+2-p2m=p2m+1-p2m+pm+2qm=-pm+1qm+pm+2qm=pm+1qm(p)=pm+1qm(p+p)=pm+1qm(p-q).
同理可得q2m+2-q2m=qm+1pm(q-p),
则q2m+2-q2m-(p2m+2-p2m)=qm+1pm(q-p)-pm+1qm(p-q)=pmqm+2-qm+1pm+1-pm+2qm+pm+1qm+1=pmqm+2-pm+2qm=pmqm(q2-p2),
因为0综上,对任意正整数m,有p2m+1-q2m+1