数学 - Python - JavaScript - 解析学 - 積分 - 積分の応用 - 確率(確率密度関数(一次関数)、付随する確率関数(二次関数))

学習環境

解析入門原書第3版 (S.ラング(著)、松坂和夫(翻訳)、片山孝次(翻訳)、岩波書店)の第3部(積分)、第13章(積分の応用)、6(確率)の練習問題2.を取り組んでみる。

定数 c を求める。

$\begin{array}{l} \int_{0}^{1} c x dx \\ = \frac{1}{2} c {[x^{2}]}_{0}^{1} \\ = \frac{1}{2} c \\ \frac{1}{2} c = 1 \\ c = 2 \end{array}$

よって、確率密度関数は、

$f (x) = 2 x^{2}$

f に付随する確率関数 F は、

$\begin{array}{l} F (x) = \int_{0}^{x} 2 t dt \\ = {[t^{2}]}_{0}^{x} \\ = x^{2} \end{array}$

求める確率は、

$\begin{array}{l} F (\frac{1}{2}) - F (0) \\ = \frac{1}{4} \end{array}$

コード(Emacs)

Python 3

#!/usr/bin/env python3
from sympy import pprint, symbols, Integral, solve, Rational, plot

print('2.')

c, x = symbols('c, x')

f = c * x

eq = Integral(f, (x, 0, 1)) - 1
cs = solve(eq.doit(), c)

for t in [eq, cs]:
    pprint(t)
    print()

fc = f.subs({c: cs[0]})

I = Integral(fc, (x, 0, Rational(1, 2)))

for t in [I, I.doit()]:
    pprint(t)
    print()

p = plot(fc, ylim=(-10, 10), legend=True, show=False)

p.save('sample2.png')

入出力結果(Terminal, Jupyter(IPython))

$ ./sample2.py
2.
1           
⌠           
⎮ c⋅x dx - 1
⌡           
0           

[2]

1/2       
 ⌠        
 ⎮  2⋅x dx
 ⌡        
 0        

1/4

$

HTML5

<div id="graph0"></div>
<pre id="output0"></pre>
<label for="r0">r = </label>
<input id="r0" type="number" min="0" value="1">
<label for="dx">dx = </label>
<input id="dx" type="number" min="0" step="0.001" value="0.1">
<br>
<label for="x1">x1 = </label>
<input id="x1" type="number" value="-2">
<label for="x2">x2 = </label>
 <input id="x2" type="number" value="2">
<br>
<label for="y1">y1 = </label>
<input id="y1" type="number" value="-2">
<label for="y2">y2 = </label>
<input id="y2" type="number" value="2">

<button id="draw0">draw</button>
<button id="clear0">clear</button>

<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/d3/4.2.6/d3.min.js" integrity="sha256-5idA201uSwHAROtCops7codXJ0vja+6wbBrZdQ6ETQc=" crossorigin="anonymous"></script>

<script src="sample2.js"></script>

JavaScript

let div0 = document.querySelector('#graph0'),
    pre0 = document.querySelector('#output0'),
    width = 600,
    height = 600,
    padding = 50,
    btn0 = document.querySelector('#draw0'),
    btn1 = document.querySelector('#clear0'),
    input_r = document.querySelector('#r0'),
    input_dx = document.querySelector('#dx'),
    input_x1 = document.querySelector('#x1'),
    input_x2 = document.querySelector('#x2'),
    input_y1 = document.querySelector('#y1'),
    input_y2 = document.querySelector('#y2'),
    inputs = [input_r, input_dx, input_x1, input_x2, input_y1, input_y2],
    p = (x) => pre0.textContent += x + '\n';

let fns = [];

let draw = () => {
    pre0.textContent = '';

    let r = parseFloat(input_r.value),
        dx = parseFloat(input_dx.value),
        x1 = parseFloat(input_x1.value),
        x2 = parseFloat(input_x2.value),
        y1 = parseFloat(input_y1.value),
        y2 = parseFloat(input_y2.value);

    if (r === 0 || dx === 0 || x1 > x2 || y1 > y2) {
        return;
    }    
    
    let points = [],
        g = x => 2 * x,
        lines = [[x1, g(x1), x2, g(x2), 'red'],
                 [x2, g(x2), x2, g(x2), 'green'],
                 [0, y1, 0, y2, 'blue'],
                 [1 / 2, y1, 1 / 2, y2, 'brown'],
                 [1, y1, 1, y2, 'orange']];

    fns
        .forEach((o) => {
            let [f, color] = o;
            for (let x = x1; x <= x2; x += dx) {
                let y = f(x);

                points.push([x, y, color]);
            }
        });
    
    let xscale = d3.scaleLinear()
        .domain([x1, x2])
        .range([padding, width - padding]);
    let yscale = d3.scaleLinear()
        .domain([y1, y2])
        .range([height - padding, padding]);

    let xaxis = d3.axisBottom().scale(xscale);
    let yaxis = d3.axisLeft().scale(yscale);
    div0.innerHTML = '';
    let svg = d3.select('#graph0')
        .append('svg')
        .attr('width', width)
        .attr('height', height);

    svg.selectAll('line')
        .data([[x1, 0, x2, 0], [0, y1, 0, y2]].concat(lines))
        .enter()
        .append('line')
        .attr('x1', (d) => xscale(d[0]))
        .attr('y1', (d) => yscale(d[1]))
        .attr('x2', (d) => xscale(d[2]))
        .attr('y2', (d) => yscale(d[3]))
        .attr('stroke', (d) => d[4] || 'black');

    svg.selectAll('circle')
        .data(points)
        .enter()
        .append('circle')
        .attr('cx', (d) => xscale(d[0]))
        .attr('cy', (d) => yscale(d[1]))
        .attr('r', r)
        .attr('fill', (d) => d[2] || 'green');

    svg.append('g')
        .attr('transform', `translate(0, ${height - padding})`)
        .call(xaxis);

    svg.append('g')
        .attr('transform', `translate(${padding}, 0)`)
        .call(yaxis);

    [fns].forEach((fs) => p(fs.join('\n')));
};

inputs.forEach((input) => input.onchange = draw);
btn0.onclick = draw;
btn1.onclick = () => pre0.textContent = '';
draw();

r = dx =
x1 = x2 =
y1 = y2 =

Kamimura's blog

2018年12月25日火曜日

数学 - Python - JavaScript - 解析学 - 積分 - 積分の応用 - 確率(確率密度関数(一次関数)、付随する確率関数(二次関数))

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