由股价服从几何布朗运动得:S(t)=S(0)*exp((u-s*s/2)*t+s*sqrt(t)*z), z~N(0,1), u=收益率期望, s=波动率,
设要求的股票收益率为r,则有 S(t)=S(0)*exp(r*t)
所以推出:r*t=(u-s*s/2)*t+s*sqrt(t)*z,即r=(u-s*s/2)+s/sqrt(t)*z
因为z~N(0,1)所以r~N((u-s*s/2),(s/sqrt(t))^2)
将u=0.15, s=0.25,t=2, 代入得r~N(11.875%,0.03125)