Poisson distribution. 1.the event are independent. 2.the event are random and unpredictable. 3.no 2 events can occur at the same time or place. 4.mean number of event is propertional to the intereval size

approximated:
binomial: np>5, nq>5
poisson: 入>20

independent：P（A∩B ）=P(A)×P(B)
Mutually exclusive：P（A∩B ）=0
complementary events：P(A)+P(B)=1

nPr=n!/(n-r)!
nCr=n!/r!(n-r)!

P(x=n) use P(n-0.5<x<n+o.5)
P(x>n) use P(x>n+o.5)
P(x>=n) use P(x>n-0.5)
P(x<n) use P(<x<n-o.5)
P(x<=n) use P(x<n+o.5)

1

relationship