begin with the the definition of the logstock return,rt+1 = log[(Pt+1+Dt+1)/Pt]。

log-linear approximation of rt+1 is given by rt+1 = k +a*pt+1 +(1-a)dt+1 -pt,where
   a =1/[1+exp(d-p)]

   k = -log(a)-(1-a)*log[(1/a)-1]


其中，(d-p)表示dt-pt的均值




怎么求出来的？求大神指教