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August 15, 2018 13:23
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"### 条件付き確率の意味を説明できる\n", | |
"\n", | |
"- No: 4\n", | |
"- Sub No: 4\n", | |
"- 難易度: ★\n", | |
"- カテゴリ名: 統計数理基礎\n", | |
"- 必須スキル: ◯\n", | |
"\n", | |
"### 条件付き確立\n", | |
"\n", | |
"事象 A と事象 B が独立である場合に、事象 B を前提とした事象 A の確立のこと。\n", | |
"\n", | |
"$$P(A|B)=\\frac{P(A \\cap B)}{P(B)}$$\n", | |
"\n", | |
"### 例題\n", | |
"\n", | |
"サイコロを 2 回投げて出た目の和が 7 以上となる確立を求める。ただし、1 回目に出た目は「3」であることが分かっている。\n", | |
"\n", | |
"![サイコロ](https://julia-for-data-science.github.io/images/4-4-saikoro.png)\n", | |
"\n", | |
"### 問題文から対象の事象を考える\n", | |
"\n", | |
"サイコロを 2 回投げて出た目の和が 7 以上となる確立を求める。ただし、1 回目に出た目は「3」であることが分かっている。\n", | |
"\n", | |
"事象 A\n", | |
"\n", | |
"2 回サイコロを投げて出た目の和が 7 以上になる\n", | |
"\n", | |
"事象 B\n", | |
"\n", | |
"1 回目にサイコロを投げた目が 3\n", | |
"\n", | |
"### $$P(A \\cap B)$$ の計算\n", | |
"\n", | |
"結合確立といって事象 A と事象 B が同時に発生する確立\n", | |
"サイコロを 2 回投げた場合の組み合わせは 36 通りで、今回の条件に当てはまる組み合わせはの次の 3 通り\n", | |
"\n", | |
"```\n", | |
"(3,4) (3,5) (3,6)\n", | |
"```\n", | |
"\n", | |
"$$P(A \\cap B)=\\frac{3}{36}$$\n", | |
"\n", | |
"### $$P(B)$$ の計算\n", | |
"\n", | |
"事象 B:1 回目にサイコロを投げた目が 3 だった事象\n", | |
"なので、\n", | |
"\n", | |
"$$P(B)=\\frac{1}{6}$$\n", | |
"\n", | |
"### $$P(A|B)=\\frac{P(A \\cap B)}{P(B)}$$ の計算\n", | |
"\n", | |
"$$P(A|B)=\\frac{\\frac{3}{36}}{\\frac{1}{6}}$$\n", | |
"\n", | |
"Julia 言語で分数のまま計算してみる\n", | |
"\n", | |
"### Julia で分数のまま計算\n", | |
"\n", | |
"Julia で分数の計算をする場合には `//` を使う。分数の四則演算や約分もしてくれる。\n", | |
"\n", | |
"```julia\n", | |
"3//36\n", | |
"=> 1//12\n", | |
"```\n", | |
"\n", | |
"#### 問題の計算\n", | |
"\n", | |
"```julia\n", | |
"(3//36) / (1//6)\n", | |
"=> 1//2\n", | |
"```\n", | |
"\n", | |
"よって\n", | |
"\n", | |
"$$P(A|B)=\\frac{\\frac{3}{36}}{\\frac{1}{6}}=\\frac{1}{2}$$\n" | |
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