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What does the math notation $\sum$ mean?
My school's prescribed book uses the weird letter E character without explaining what it is in the first chapter when it talks about the binomial equation. I can't find it on Google either because I don't know what it means or its name. Please help me!
$ (x+a)^n = \sum_{k=0}^{n} \binom{n}{k}x^ka^{n-k}$