## 2018年12月1日土曜日

### 数学 - Python - JavaScript - 解析学 - 積分 - 積分の応用 - 回転体の体積(2次関数と直線に挟まれる領域、x軸の周りに回転、定積分)

1. $\begin{array}{}{x}^{2}=5x=0\\ {x}^{2}-5x=0\\ x\left(x-5\right)=0\\ x=0,5\\ 0\le x\le 5\\ 5x-{x}^{2}\ge 0\end{array}$

よって、求める2次関数と直線にはさまれる領域を x軸のまわりに回転してできる回転体の体積は、

$\begin{array}{}{\int }_{0}^{5}\pi {\left(5x\right)}^{2}\mathrm{dx}-{\int }_{0}^{5}\pi {\left({x}^{2}\right)}^{2}\mathrm{dx}\\ =\pi {\int }_{0}^{5}\left({5}^{2}{x}^{2}-{x}^{4}\right)\mathrm{dx}\\ =\pi {\left[\frac{{5}^{2}}{3}{x}^{3}-\frac{1}{5}{x}^{5}\right]}_{0}^{5}\\ =\pi \left(\frac{{5}^{2}}{3}·{5}^{3}-\frac{1}{5}·{5}^{5}\right)\\ =\frac{\pi \left({5}^{5}-3·{5}^{4}\right)}{3}\\ =\frac{2·{5}^{4}\pi }{3}\end{array}$

コード(Emacs)

Python 3

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

print('5.')
x = symbols('x')
f = 5 * x
g = x ** 2
x1, x2 = sorted(solve(f - g, x))

I = Integral(pi * f ** 2, (x, x1, x2)) - Integral(pi * g ** 2, (x, x1, x2))

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

p = plot(f, g, show=False, legend=True)
colors = ['red', 'green']
for i, color in enumerate(colors):
p[i].line_color = color

p.save('sample5.svg')


$./sample5.py 5. 5 5 ⌠ ⌠ ⎮ 4 ⎮ 2 - ⎮ π⋅x dx + ⎮ 25⋅π⋅x dx ⌡ ⌡ 0 0 1250⋅π ────── 3$


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="0.01">
<br>
<label for="x1">x1 = </label>
<input id="x1" type="number" value="-25">
<label for="x2">x2 = </label>
<input id="x2" type="number" value="25">
<br>
<label for="y1">y1 = </label>
<input id="y1" type="number" value="-25">
<label for="y2">y2 = </label>
<input id="y2" type="number" value="25">

<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="sample5.js"></script>


JavaScript

let div0 = document.querySelector('#graph0'),
pre0 = document.querySelector('#output0'),
width = 600,
height = 600,
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 => x ** 2, 'red'],
[x => 5 * x, 'green']];

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 = [[0, y1, 0, y2, 'blue'],
[5, y1, 5, 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])
let yscale = d3.scaleLinear()
.domain([y1, y2])

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();