Lengths of randomly broken rods
$begingroup$
A continuous random variable $X$ has probability density function $f(x)$. The probability that $X leq x$ is given by the function $F(x)$.
i) Explain why $F'(x) = f(x)$.
ii) A rod of length $2a$ is broken into two parts at a point whose position is random. State the form of the probability distribution of the length of the smaller part.
iii) Find the mean value of this length.
iv) Two equal rods, each of length $2a$, are broken into two parts at points whose positions are random. $X$ is the length of the shortest of the four parts thus obtained. Find the probability, $F(x)$, that $X leq x$, where $0 < x leq a$.
v) Hence or otherwise, show that the probability density function of $X$ is given by
$$f(x) = begin{cases} (2(a-x))/a^2 & 0 < x leq a, \ 0 & x leq 0 text{ or } x > a. end{cases}$$
vi) Show that the mean value of $X$ is $a/3$.
vii) Find the mean value of the sum of the two smaller parts and hence show the mean values of the four parts are in the proportions $1:2:4:5$.
probability random-variables
$endgroup$
add a comment |
$begingroup$
A continuous random variable $X$ has probability density function $f(x)$. The probability that $X leq x$ is given by the function $F(x)$.
i) Explain why $F'(x) = f(x)$.
ii) A rod of length $2a$ is broken into two parts at a point whose position is random. State the form of the probability distribution of the length of the smaller part.
iii) Find the mean value of this length.
iv) Two equal rods, each of length $2a$, are broken into two parts at points whose positions are random. $X$ is the length of the shortest of the four parts thus obtained. Find the probability, $F(x)$, that $X leq x$, where $0 < x leq a$.
v) Hence or otherwise, show that the probability density function of $X$ is given by
$$f(x) = begin{cases} (2(a-x))/a^2 & 0 < x leq a, \ 0 & x leq 0 text{ or } x > a. end{cases}$$
vi) Show that the mean value of $X$ is $a/3$.
vii) Find the mean value of the sum of the two smaller parts and hence show the mean values of the four parts are in the proportions $1:2:4:5$.
probability random-variables
$endgroup$
$begingroup$
Question i is the only question I could do. Got really stuck on the other parts. Any hints of what to do?
$endgroup$
– user624037
Dec 7 '18 at 21:43
$begingroup$
Do you know what a uniform distribution is?
$endgroup$
– Trurl
Dec 7 '18 at 22:20
$begingroup$
Yes, and i'm pretty sure the answer to ii) is a uniform distribution. But I don't understand why
$endgroup$
– user624037
Dec 7 '18 at 22:25
add a comment |
$begingroup$
A continuous random variable $X$ has probability density function $f(x)$. The probability that $X leq x$ is given by the function $F(x)$.
i) Explain why $F'(x) = f(x)$.
ii) A rod of length $2a$ is broken into two parts at a point whose position is random. State the form of the probability distribution of the length of the smaller part.
iii) Find the mean value of this length.
iv) Two equal rods, each of length $2a$, are broken into two parts at points whose positions are random. $X$ is the length of the shortest of the four parts thus obtained. Find the probability, $F(x)$, that $X leq x$, where $0 < x leq a$.
v) Hence or otherwise, show that the probability density function of $X$ is given by
$$f(x) = begin{cases} (2(a-x))/a^2 & 0 < x leq a, \ 0 & x leq 0 text{ or } x > a. end{cases}$$
vi) Show that the mean value of $X$ is $a/3$.
vii) Find the mean value of the sum of the two smaller parts and hence show the mean values of the four parts are in the proportions $1:2:4:5$.
probability random-variables
$endgroup$
A continuous random variable $X$ has probability density function $f(x)$. The probability that $X leq x$ is given by the function $F(x)$.
i) Explain why $F'(x) = f(x)$.
ii) A rod of length $2a$ is broken into two parts at a point whose position is random. State the form of the probability distribution of the length of the smaller part.
iii) Find the mean value of this length.
iv) Two equal rods, each of length $2a$, are broken into two parts at points whose positions are random. $X$ is the length of the shortest of the four parts thus obtained. Find the probability, $F(x)$, that $X leq x$, where $0 < x leq a$.
v) Hence or otherwise, show that the probability density function of $X$ is given by
$$f(x) = begin{cases} (2(a-x))/a^2 & 0 < x leq a, \ 0 & x leq 0 text{ or } x > a. end{cases}$$
vi) Show that the mean value of $X$ is $a/3$.
vii) Find the mean value of the sum of the two smaller parts and hence show the mean values of the four parts are in the proportions $1:2:4:5$.
probability random-variables
probability random-variables
edited Dec 7 '18 at 22:16
JHF
4,7111026
4,7111026
asked Dec 7 '18 at 21:43
user624037
$begingroup$
Question i is the only question I could do. Got really stuck on the other parts. Any hints of what to do?
$endgroup$
– user624037
Dec 7 '18 at 21:43
$begingroup$
Do you know what a uniform distribution is?
$endgroup$
– Trurl
Dec 7 '18 at 22:20
$begingroup$
Yes, and i'm pretty sure the answer to ii) is a uniform distribution. But I don't understand why
$endgroup$
– user624037
Dec 7 '18 at 22:25
add a comment |
$begingroup$
Question i is the only question I could do. Got really stuck on the other parts. Any hints of what to do?
$endgroup$
– user624037
Dec 7 '18 at 21:43
$begingroup$
Do you know what a uniform distribution is?
$endgroup$
– Trurl
Dec 7 '18 at 22:20
$begingroup$
Yes, and i'm pretty sure the answer to ii) is a uniform distribution. But I don't understand why
$endgroup$
– user624037
Dec 7 '18 at 22:25
$begingroup$
Question i is the only question I could do. Got really stuck on the other parts. Any hints of what to do?
$endgroup$
– user624037
Dec 7 '18 at 21:43
$begingroup$
Question i is the only question I could do. Got really stuck on the other parts. Any hints of what to do?
$endgroup$
– user624037
Dec 7 '18 at 21:43
$begingroup$
Do you know what a uniform distribution is?
$endgroup$
– Trurl
Dec 7 '18 at 22:20
$begingroup$
Do you know what a uniform distribution is?
$endgroup$
– Trurl
Dec 7 '18 at 22:20
$begingroup$
Yes, and i'm pretty sure the answer to ii) is a uniform distribution. But I don't understand why
$endgroup$
– user624037
Dec 7 '18 at 22:25
$begingroup$
Yes, and i'm pretty sure the answer to ii) is a uniform distribution. But I don't understand why
$endgroup$
– user624037
Dec 7 '18 at 22:25
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Let $U = U(0,2a)$ be a uniform distribution supported on the interval $(0,2a)$.
For (ii), let $Y$ be the length of the shorter part of the broken rod. Then $Y = min{U, 2a - U}$. So, for $0 < x leq a$,
begin{align}
mathbb{P}(Y leq x) &= mathbb{P}(min{U, 2a - U} leq x) = mathbb{P}(U leq x text{ or } 2a - U leq x)\
&= mathbb{P}(U leq x) + mathbb{P}(2a - U leq x) - mathbb{P}(2a - x leq U leq x) \
&= frac{x}{2a} + frac{x}{2a} - 0 = frac{x}{a}.
end{align}
So $Y sim U(0,a)$ is uniformly distributed on $(0,a)$.
Part (iii) should be standard given part (ii), since if $f_Y(x)$ is the pdf for $Y$, then $$mathbb{E}Y = int_0^a x f_Y(x) , dx,$$ and I'll leave the details to you.
For (iv), let $Y_1$ be the shorter part of the first broken rod and let $Y_2$ be the shorter part of the second broken rod. Then $Y_1$ and $Y_2$ are iid with distribution $Y$, and $X = min{Y_1, Y_2}$. So,
begin{align*}
mathbb{P}(X leq x) &= mathbb{P}(Y_1 leq x text{ or } Y_2 leq x) \
&= mathbb{P}(Y_1 leq x) + mathbb{P}(Y_2 leq x) - mathbb{P}(Y_1 leq x) mathbb{P}(Y_2 leq x) \
&= frac{x}{a} + frac{x}{a} - (frac{x}{a})^2 = frac{2ax-x^2}{a^2}.
end{align*}
Using part (i), you should be able to derive the stated equation. I'll again leave part (v) to you.
Finally, for (vi), use the fact that expectation is additive, so that $mathbb{E}(Y_1 + Y_2) = mathbb{E}Y_1 + mathbb{E}Y_2$, and proceed.
$endgroup$
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3030393%2flengths-of-randomly-broken-rods%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Let $U = U(0,2a)$ be a uniform distribution supported on the interval $(0,2a)$.
For (ii), let $Y$ be the length of the shorter part of the broken rod. Then $Y = min{U, 2a - U}$. So, for $0 < x leq a$,
begin{align}
mathbb{P}(Y leq x) &= mathbb{P}(min{U, 2a - U} leq x) = mathbb{P}(U leq x text{ or } 2a - U leq x)\
&= mathbb{P}(U leq x) + mathbb{P}(2a - U leq x) - mathbb{P}(2a - x leq U leq x) \
&= frac{x}{2a} + frac{x}{2a} - 0 = frac{x}{a}.
end{align}
So $Y sim U(0,a)$ is uniformly distributed on $(0,a)$.
Part (iii) should be standard given part (ii), since if $f_Y(x)$ is the pdf for $Y$, then $$mathbb{E}Y = int_0^a x f_Y(x) , dx,$$ and I'll leave the details to you.
For (iv), let $Y_1$ be the shorter part of the first broken rod and let $Y_2$ be the shorter part of the second broken rod. Then $Y_1$ and $Y_2$ are iid with distribution $Y$, and $X = min{Y_1, Y_2}$. So,
begin{align*}
mathbb{P}(X leq x) &= mathbb{P}(Y_1 leq x text{ or } Y_2 leq x) \
&= mathbb{P}(Y_1 leq x) + mathbb{P}(Y_2 leq x) - mathbb{P}(Y_1 leq x) mathbb{P}(Y_2 leq x) \
&= frac{x}{a} + frac{x}{a} - (frac{x}{a})^2 = frac{2ax-x^2}{a^2}.
end{align*}
Using part (i), you should be able to derive the stated equation. I'll again leave part (v) to you.
Finally, for (vi), use the fact that expectation is additive, so that $mathbb{E}(Y_1 + Y_2) = mathbb{E}Y_1 + mathbb{E}Y_2$, and proceed.
$endgroup$
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
add a comment |
$begingroup$
Let $U = U(0,2a)$ be a uniform distribution supported on the interval $(0,2a)$.
For (ii), let $Y$ be the length of the shorter part of the broken rod. Then $Y = min{U, 2a - U}$. So, for $0 < x leq a$,
begin{align}
mathbb{P}(Y leq x) &= mathbb{P}(min{U, 2a - U} leq x) = mathbb{P}(U leq x text{ or } 2a - U leq x)\
&= mathbb{P}(U leq x) + mathbb{P}(2a - U leq x) - mathbb{P}(2a - x leq U leq x) \
&= frac{x}{2a} + frac{x}{2a} - 0 = frac{x}{a}.
end{align}
So $Y sim U(0,a)$ is uniformly distributed on $(0,a)$.
Part (iii) should be standard given part (ii), since if $f_Y(x)$ is the pdf for $Y$, then $$mathbb{E}Y = int_0^a x f_Y(x) , dx,$$ and I'll leave the details to you.
For (iv), let $Y_1$ be the shorter part of the first broken rod and let $Y_2$ be the shorter part of the second broken rod. Then $Y_1$ and $Y_2$ are iid with distribution $Y$, and $X = min{Y_1, Y_2}$. So,
begin{align*}
mathbb{P}(X leq x) &= mathbb{P}(Y_1 leq x text{ or } Y_2 leq x) \
&= mathbb{P}(Y_1 leq x) + mathbb{P}(Y_2 leq x) - mathbb{P}(Y_1 leq x) mathbb{P}(Y_2 leq x) \
&= frac{x}{a} + frac{x}{a} - (frac{x}{a})^2 = frac{2ax-x^2}{a^2}.
end{align*}
Using part (i), you should be able to derive the stated equation. I'll again leave part (v) to you.
Finally, for (vi), use the fact that expectation is additive, so that $mathbb{E}(Y_1 + Y_2) = mathbb{E}Y_1 + mathbb{E}Y_2$, and proceed.
$endgroup$
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
add a comment |
$begingroup$
Let $U = U(0,2a)$ be a uniform distribution supported on the interval $(0,2a)$.
For (ii), let $Y$ be the length of the shorter part of the broken rod. Then $Y = min{U, 2a - U}$. So, for $0 < x leq a$,
begin{align}
mathbb{P}(Y leq x) &= mathbb{P}(min{U, 2a - U} leq x) = mathbb{P}(U leq x text{ or } 2a - U leq x)\
&= mathbb{P}(U leq x) + mathbb{P}(2a - U leq x) - mathbb{P}(2a - x leq U leq x) \
&= frac{x}{2a} + frac{x}{2a} - 0 = frac{x}{a}.
end{align}
So $Y sim U(0,a)$ is uniformly distributed on $(0,a)$.
Part (iii) should be standard given part (ii), since if $f_Y(x)$ is the pdf for $Y$, then $$mathbb{E}Y = int_0^a x f_Y(x) , dx,$$ and I'll leave the details to you.
For (iv), let $Y_1$ be the shorter part of the first broken rod and let $Y_2$ be the shorter part of the second broken rod. Then $Y_1$ and $Y_2$ are iid with distribution $Y$, and $X = min{Y_1, Y_2}$. So,
begin{align*}
mathbb{P}(X leq x) &= mathbb{P}(Y_1 leq x text{ or } Y_2 leq x) \
&= mathbb{P}(Y_1 leq x) + mathbb{P}(Y_2 leq x) - mathbb{P}(Y_1 leq x) mathbb{P}(Y_2 leq x) \
&= frac{x}{a} + frac{x}{a} - (frac{x}{a})^2 = frac{2ax-x^2}{a^2}.
end{align*}
Using part (i), you should be able to derive the stated equation. I'll again leave part (v) to you.
Finally, for (vi), use the fact that expectation is additive, so that $mathbb{E}(Y_1 + Y_2) = mathbb{E}Y_1 + mathbb{E}Y_2$, and proceed.
$endgroup$
Let $U = U(0,2a)$ be a uniform distribution supported on the interval $(0,2a)$.
For (ii), let $Y$ be the length of the shorter part of the broken rod. Then $Y = min{U, 2a - U}$. So, for $0 < x leq a$,
begin{align}
mathbb{P}(Y leq x) &= mathbb{P}(min{U, 2a - U} leq x) = mathbb{P}(U leq x text{ or } 2a - U leq x)\
&= mathbb{P}(U leq x) + mathbb{P}(2a - U leq x) - mathbb{P}(2a - x leq U leq x) \
&= frac{x}{2a} + frac{x}{2a} - 0 = frac{x}{a}.
end{align}
So $Y sim U(0,a)$ is uniformly distributed on $(0,a)$.
Part (iii) should be standard given part (ii), since if $f_Y(x)$ is the pdf for $Y$, then $$mathbb{E}Y = int_0^a x f_Y(x) , dx,$$ and I'll leave the details to you.
For (iv), let $Y_1$ be the shorter part of the first broken rod and let $Y_2$ be the shorter part of the second broken rod. Then $Y_1$ and $Y_2$ are iid with distribution $Y$, and $X = min{Y_1, Y_2}$. So,
begin{align*}
mathbb{P}(X leq x) &= mathbb{P}(Y_1 leq x text{ or } Y_2 leq x) \
&= mathbb{P}(Y_1 leq x) + mathbb{P}(Y_2 leq x) - mathbb{P}(Y_1 leq x) mathbb{P}(Y_2 leq x) \
&= frac{x}{a} + frac{x}{a} - (frac{x}{a})^2 = frac{2ax-x^2}{a^2}.
end{align*}
Using part (i), you should be able to derive the stated equation. I'll again leave part (v) to you.
Finally, for (vi), use the fact that expectation is additive, so that $mathbb{E}(Y_1 + Y_2) = mathbb{E}Y_1 + mathbb{E}Y_2$, and proceed.
answered Dec 7 '18 at 22:55
JHFJHF
4,7111026
4,7111026
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
add a comment |
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Okay thank you. Am just stuck on the last part. How can I show that the mean values of the four parts are in the proportions 1: 2: 4 : 5?
$endgroup$
– user624037
Dec 7 '18 at 23:46
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
$begingroup$
Since you know the mean length of the shortest rod out of the four is $frac{a}{3}$, and the mean of the sum of the shortest two from (vii), what is the mean length of the second shortest rod? If you have a broken rod of total length $2a$ and the shorter part has, say, mean length $frac{a}{3}$, what is the mean length of the longer part?
$endgroup$
– JHF
Dec 8 '18 at 23:10
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3030393%2flengths-of-randomly-broken-rods%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
$begingroup$
Question i is the only question I could do. Got really stuck on the other parts. Any hints of what to do?
$endgroup$
– user624037
Dec 7 '18 at 21:43
$begingroup$
Do you know what a uniform distribution is?
$endgroup$
– Trurl
Dec 7 '18 at 22:20
$begingroup$
Yes, and i'm pretty sure the answer to ii) is a uniform distribution. But I don't understand why
$endgroup$
– user624037
Dec 7 '18 at 22:25