1. I this from my hw solution. (1-t/s)^n = exp(-t/s) as n goes to infinity I don't understand. I checked the exponential power series. It should be : exp(x) = summation (x^n / n!) n=0 to infinity How come it could be a exponential function ? 2. another is that why <t> = integral from 0 to infinity (t*P(t) dt) ? average t P(t)dt = probability that an electron has no colission till time t * probability that it has a collision between time t probablitiy has no collision is exp(-t/s) t+dt = exp(-t/s) *dt/s Thanks a lot !