I'm stuck. Please help. Thanks.
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$\displaystyle \sum\limits_{j = 0}^n {{j+k-1}\choose j} $
Originally Posted by qtpipi I'm stuck. Please help. Thanks. Hint: Notice that if $\displaystyle x_1 + \dots + x_k \leq n$ where $\displaystyle 0 \leq x_i$ for all i, then $\displaystyle x_1 + \dots +x_k + x_{k+1} = n$ where $\displaystyle 0 \leq x_i$ for all i.
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