This is the question of combination.

I have tried this but how can the binomial expansion can result our need? How we can proof the formula:

The number of ways in which n different things can be distributed into $r$ different groups is coefficient of $x^n$ in $n!(e^x-1)^r$?

  • 1
    $\begingroup$ $n!(e^x-1)$ is an expression that is independent of $r$, are you sure you stated the question correctly? math.meta.stackexchange.com/q/5020/306553 also, learn mathjax to type maths on this site. $\endgroup$ – Siong Thye Goh Sep 6 '17 at 5:32
  • $\begingroup$ @SiongThyeGoh , there's an $r$ in the expression, maybe it has been edited since you originally commented. $\endgroup$ – TeeJay Sep 6 '17 at 6:33
  • $\begingroup$ Yes...I have edited that..after siong Thye Goh commented $\endgroup$ – Kazi Abu Rousan Sep 6 '17 at 9:45

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