Untitled

計算の天才マーティン・ガードナーは777の二乗を計算するとき、まず777に23を足して計算しやすい800にした。彼は100までの二乗ならば答えを暗記していたので23の二乗は529だとすぐに分かった。

そこで、

（777+23）×（777-23）＋529

＝（800×754）＋529

を計算し、603729という答えを瞬時に計算した。

800×754は3桁の掛け算だが、実際には8×754を100倍するだけなので暗算も易しい。方法は違いそうだが、ガードナーは5桁の掛け算も似たようなトリックで、瞬時に計算できたらしい。

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— Passion For The Future: なぜ数学が「得意な人」と「苦手な人」がいるのか (via otsune)
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