Für genügend großes n gilt doch
\(\sum \limits_{k=0}^{n}(2\sqrt{k}-4\sqrt{k+1}+2\sqrt{k+2})\)
\(= \sum \limits_{k=0}^{n}2\sqrt{k}-\sum \limits_{k=0}^{n}4\sqrt{k+1}+\sum \limits_{k=0}^{n}2\sqrt{k+2}\)
\(= \sum \limits_{k=0}^{n}2\sqrt{k}-\sum \limits_{k=1}^{n+1}4\sqrt{k}+\sum \limits_{k=2}^{n+2}2\sqrt{k}\)
\(= 2+\sum \limits_{k=2}^{n}2\sqrt{k}-4-\sum \limits_{k=2}^{n}4\sqrt{k}-4\sqrt{n+1}+\sum \limits_{k=2}^{n}2\sqrt{k}+2\sqrt{n+1}+2\sqrt{n+2}\)
\(= 2-4-4\sqrt{n+1}+2\sqrt{n+1}+2\sqrt{n+2}+\sum \limits_{k=2}^{n}2\sqrt{k}-\sum \limits_{k=2}^{n}4\sqrt{k}+\sum \limits_{k=2}^{n}2\sqrt{k}\)
\(= 2-4-4\sqrt{n+1}+2\sqrt{n+1}+2\sqrt{n+2}+\sum \limits_{k=2}^{n}(2\sqrt{k}-4\sqrt{k}+2\sqrt{k})\)
\(= 2-4-4\sqrt{n+1}+2\sqrt{n+1}+2\sqrt{n+2}\)
\(= -2-2\sqrt{n+1}+2\sqrt{n+2}\)