E(number of trials until HH)=(1-p^2) / (q*p^2) = (1+p)/(p^2)
E(number of trials until TT)=(1-q^2) / (p*q^2) = (1+q)/(q^2)
=> rate of HH = p^2 / (1+p)
rate of TT =q^2 / (1+q) = (1-p)^2 / (2-p)
=> P(HHTT)= [p^2/(1+p)] / {[p^2/(1+p)] + [(1-p)^2 / (2-p)]}
希望高手给检查下,谢谢!