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2006年02月08日

乙π

昨日の続きOpen@NewWindow

自分なりに色々調べてみた。
他所の処理系がどうなのかは知らないけれど、Visual Stdio .net 2003付属のC/C++にもπの定数があった。しかもmath.hに定義してやがんのな。しかしそれは_USE_MATH_DEFINESをインクルード前に定義しなければ利用できないわけで。

arctan(1)×4≒π
については、僕が理解した範囲で二種類の解き方がある。

1つめ。
arctan(1)というのは、内角がそれぞれ90度、45度、45度で構成された二等辺直角三角形のことだ。
なので、arctan(1)というのは、角度単位系で言えば45度のこと。
45度×4で180度=πラジアン=3.141592…という帰結。

2つめ。
arctan(x)をテイラー展開するOpen@NewWindow
このxに1を代入するとπ/4の近似値になる。それに4を掛けることでπだけを取り出せる。
パソコンで無理数を扱うことは出来ないので、sin、cosなんかもテイラー展開やそれに類する式で実現していることは想像に難くない。(ソースコードやANSIの定義書を探すのがダルイのでここでは追求しない)
ちなみに僕が先に理解できたのは、こっちのほうでした。

よって、arctan(1)×4≒π
と言える。C++の倍精度浮動小数点型で計算したところ、得られたπの値は小数点以下15桁まで有効な値でした。16桁目で誤差が出ちゃってた。

この検証によって判ったのは
・僕の頭は非常に飲み込みが悪い上、数学のセンスが著しく欠けている
・もうarctan(1)×4≒πは死ぬまで忘れないだろう
ということ。

お、なんか博士の愛した数式Open@NewWindowみたいになってきました。読んでないけど。あと、タイトルの読みは気にしなくてよろしい。

コメント

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