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| rev | Admin | (Created page with "Prove that <math display="block"> \sum_{j,k} \biggl(\frac 1{3^n}\frac{n!}{j!k!(n-j-k)!}\biggr) = 1\ , </math> where the sum extends over all non-negative <math>j</math> and <math>k</math> such that <math>j + k \le n</math>. '' Hint'': Count how many ways one can place <math>n</math> labelled balls in 3 labelled urns.") | Jun 15'24 at 2:47 | +321 |