I have just made the following variable switch:(adsbygoogle = window.adsbygoogle || []).push({});

[tex]\sum_{i=0}^n\sum_{j=0}^m\binom{n}{i}\binom{m}{ j}x^{i+j}=\sum_{k=0}^{n+m}\sum_{i=0}^k\binom{n}{i}\binom{m}{k-i}x^{k}[/tex]

I know it's right, but is there a method I can use to prove without a shadow of a doubt that it is?

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# Homework Help: Changing the summation indexes in double sums.

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