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**HD09** Is there an intuitive way to explain the forumla $\displaystyle P(X=x)=\frac{e^{-\mu}\mu^x}{x!}$ ?

For example in the binomial distribution with n trials & probability of sucess p you can see that to get r sucesses you must also have n-r failures and the trials are independent so this probability is p^r x (1-p)^n-r, and then you don't care about the order so that's where the n choose r comes in...

But is there a way to explain the poisson formula? Where did it come from?