数列の和
数列の和といえば,部分分数分解を思い浮かべる人が多いと思います.甘いです.もう1パターンあります.
問題
$\displaystyle\sum_{k=1}^{n}\displaystyle\frac{1}{\sqrt{k+3}+\sqrt{k+2}}$を求めよ.
解答
$\displaystyle\sum_{k=1}^{n}\displaystyle\frac{1}{\sqrt{k+3}+\sqrt{k+2}}=\displaystyle\sum_{k=1}^{n}\displaystyle\frac{\sqrt{k+3}-\sqrt{k+2}}{(\sqrt{k+3}+\sqrt{k+2})(\sqrt{k+3}-\sqrt{k+2})}$
$=\displaystyle\sum_{k=1}^{n}\displaystyle\frac{\sqrt{k+3}-\sqrt{k+2}}{(k+3)-(k+2)}$
$=\displaystyle\sum_{k=1}^{n}(\sqrt{k+3}-\sqrt{k+2})$
$=\sqrt{n+3}-\sqrt{3}$ (答)
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