数学 - Python - JavaScript - 解析学 - 積分 - 積分の応用 - 確率(絶対値、三角関数(余弦)、確率密度関数、付随する確率関数)

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9.の解答を求めてみる。

定数 c を求める。

$\begin{array}{l} \int_{- π}^{π} c |\sin x| dx \\ = 2 c \int_{0}^{π} \sin x dx \\ = - 2 c {[\cos x]}_{0}^{π} \\ = - 2 c (- 1 - 1) \\ = 4 c \\ 4 c = 1 \\ c = \frac{1}{4} \end{array}$

確率密度関数。

$f (x) = \frac{1}{4} |\sin x|$

f に付随する確率関数。

$\begin{array}{l} f (x) \\ = \int_{- π}^{π} \frac{1}{4} |\sin x| dx \end{array}$

求める確率。

$\begin{array}{l} \int_{0}^{\frac{π}{2}} \frac{1}{4} |\sin x| dx \\ = \frac{1}{4} {[- \cos x]}_{0}^{\frac{π}{2}} \\ = \frac{1}{4} \end{array}$

コード(Emacs)

Python 3

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

print('9.')

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

f = c * abs(sin(x))

eq = Integral(f, (x, -pi, pi)) - 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, pi / 2))

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

p = plot((fc, (x, -pi, 0)),
         (fc, (x, 0, pi / 2)),
         (fc, (x, pi / 2, pi)),
         legend=True, show=False)
colors = ['red', 'green', 'red']
for i, color in enumerate(colors):
    p[i].line_color = color
p.save('sample9.png')

入出力結果(Terminal, Jupyter(IPython))

$ ./sample9.py
9.
π                   
⌠                   
⎮  c⋅│sin(x)│ dx - 1
⌡                   
-π                  

[1/4]

π            
─            
2            
⌠            
⎮ │sin(x)│   
⎮ ──────── dx
⎮    4       
⌡            
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="0.001">
<br>
<label for="x1">x1 = </label>
<input id="x1" type="number" value="-5">
<label for="x2">x2 = </label>
 <input id="x2" type="number" value="5">
<br>
<label for="y1">y1 = </label>
<input id="y1" type="number" value="-5">
<label for="y2">y2 = </label>
<input id="y2" type="number" value="5">

<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="sample9.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 = [[x => 1 / 4 * Math.abs(Math.sin(x)), 'red']];

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 = [],
        lines = [[-Math.PI, y1, -Math.PI, y2, 'green'],
                 [0, y1, 0, y2, 'blue'],
                 [Math.PI / 2, y1, Math.PI / 2, y2, 'brown'],
                 [Math.PI, y1, Math.PI, 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

2019年1月4日金曜日

数学 - Python - JavaScript - 解析学 - 積分 - 積分の応用 - 確率(絶対値、三角関数(余弦)、確率密度関数、付随する確率関数)

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