Joint PDF P[X+Y<=0.5]
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I am having some difficulty with this probability that I need to compute, and I could use some help from other eyes to see where I am messing up:
Joint PDF: $x+y$ for $0 < x < 1$, $0 < y < 1$.
I need to find the $P(x+yleq 0.5)$.
For the double integration, I have the following bounds:
Outer bound is respectively to $x$ and is from $0$ to $0.5$.
Inner bound is respective to $y$ and is from $0$ to $0.5 - x$.
Here is the initial set up:
$int_0^.5 int_0^{.5-x}(x +y) dydx$.
My steps of integration:
1: $int_0^.5 left(xy + frac{y^2}{2}right)Big|_0^{.5-x}dx$.
2: $int_0^.5$ $left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx $.
3: $(frac{x^2}{4} - frac{x^3}{3} + frac{(0.5-x)^3}{6})Big|_0^{0.5}$.
I am getting a negative number based on this final equation, and the answer should be $1/24$. Can someone help me figure out where I went wrong?
probability
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add a comment |
$begingroup$
I am having some difficulty with this probability that I need to compute, and I could use some help from other eyes to see where I am messing up:
Joint PDF: $x+y$ for $0 < x < 1$, $0 < y < 1$.
I need to find the $P(x+yleq 0.5)$.
For the double integration, I have the following bounds:
Outer bound is respectively to $x$ and is from $0$ to $0.5$.
Inner bound is respective to $y$ and is from $0$ to $0.5 - x$.
Here is the initial set up:
$int_0^.5 int_0^{.5-x}(x +y) dydx$.
My steps of integration:
1: $int_0^.5 left(xy + frac{y^2}{2}right)Big|_0^{.5-x}dx$.
2: $int_0^.5$ $left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx $.
3: $(frac{x^2}{4} - frac{x^3}{3} + frac{(0.5-x)^3}{6})Big|_0^{0.5}$.
I am getting a negative number based on this final equation, and the answer should be $1/24$. Can someone help me figure out where I went wrong?
probability
$endgroup$
add a comment |
$begingroup$
I am having some difficulty with this probability that I need to compute, and I could use some help from other eyes to see where I am messing up:
Joint PDF: $x+y$ for $0 < x < 1$, $0 < y < 1$.
I need to find the $P(x+yleq 0.5)$.
For the double integration, I have the following bounds:
Outer bound is respectively to $x$ and is from $0$ to $0.5$.
Inner bound is respective to $y$ and is from $0$ to $0.5 - x$.
Here is the initial set up:
$int_0^.5 int_0^{.5-x}(x +y) dydx$.
My steps of integration:
1: $int_0^.5 left(xy + frac{y^2}{2}right)Big|_0^{.5-x}dx$.
2: $int_0^.5$ $left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx $.
3: $(frac{x^2}{4} - frac{x^3}{3} + frac{(0.5-x)^3}{6})Big|_0^{0.5}$.
I am getting a negative number based on this final equation, and the answer should be $1/24$. Can someone help me figure out where I went wrong?
probability
$endgroup$
I am having some difficulty with this probability that I need to compute, and I could use some help from other eyes to see where I am messing up:
Joint PDF: $x+y$ for $0 < x < 1$, $0 < y < 1$.
I need to find the $P(x+yleq 0.5)$.
For the double integration, I have the following bounds:
Outer bound is respectively to $x$ and is from $0$ to $0.5$.
Inner bound is respective to $y$ and is from $0$ to $0.5 - x$.
Here is the initial set up:
$int_0^.5 int_0^{.5-x}(x +y) dydx$.
My steps of integration:
1: $int_0^.5 left(xy + frac{y^2}{2}right)Big|_0^{.5-x}dx$.
2: $int_0^.5$ $left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx $.
3: $(frac{x^2}{4} - frac{x^3}{3} + frac{(0.5-x)^3}{6})Big|_0^{0.5}$.
I am getting a negative number based on this final equation, and the answer should be $1/24$. Can someone help me figure out where I went wrong?
probability
probability
edited Dec 5 '18 at 10:42
MitterHai
asked Dec 5 '18 at 10:19
MitterHaiMitterHai
84
84
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add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Your made the sign error in step 2 (as indicated by KaviRamaMurthy).
As an alternative, you can simplify in step 2 before integrating:
$$int_0^{0.5}left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx=
int_0^{0.5}left(require{cancel}cancel{frac{x}{2}} -x^2+frac{1}{8}-cancel{frac x2}+frac{x^2}{2}right) dx=\
int_0^{0.5}left(-frac{x^2}{2}+frac18right) dx=
left(-frac{x^3}{6}+frac x8right)|_0^{0.5}=
-frac{1}{48}+frac{1}{16}=
frac 1{24}.$$
Answering your comment, yes, for the upper limit $0.5$ the term will be $0$, but for the lower limit $0$, the term will be $frac1{48}$:
$$(frac{x^2}{4} - frac{x^3}{3} color{red}{-} frac{(0.5-x)^3}{6})Big|_0^{0.5}=left(frac{1}{16}-frac1{24}-0right)-left(0-0-frac1{48}right)=frac1{24}.$$
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Farruhota, thank you for the awesome feedback. I really appreciate your help.
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– MitterHai
Dec 5 '18 at 11:42
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You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
add a comment |
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Anti-derivative of $frac {(0.5-x)^{2}} 2$ is $-frac {(0.5-x)^{3}} 6$. You are missing the minus sign.
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Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
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– MitterHai
Dec 5 '18 at 10:38
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@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
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What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
add a comment |
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2 Answers
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active
oldest
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2 Answers
2
active
oldest
votes
active
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votes
$begingroup$
Your made the sign error in step 2 (as indicated by KaviRamaMurthy).
As an alternative, you can simplify in step 2 before integrating:
$$int_0^{0.5}left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx=
int_0^{0.5}left(require{cancel}cancel{frac{x}{2}} -x^2+frac{1}{8}-cancel{frac x2}+frac{x^2}{2}right) dx=\
int_0^{0.5}left(-frac{x^2}{2}+frac18right) dx=
left(-frac{x^3}{6}+frac x8right)|_0^{0.5}=
-frac{1}{48}+frac{1}{16}=
frac 1{24}.$$
Answering your comment, yes, for the upper limit $0.5$ the term will be $0$, but for the lower limit $0$, the term will be $frac1{48}$:
$$(frac{x^2}{4} - frac{x^3}{3} color{red}{-} frac{(0.5-x)^3}{6})Big|_0^{0.5}=left(frac{1}{16}-frac1{24}-0right)-left(0-0-frac1{48}right)=frac1{24}.$$
$endgroup$
$begingroup$
Farruhota, thank you for the awesome feedback. I really appreciate your help.
$endgroup$
– MitterHai
Dec 5 '18 at 11:42
$begingroup$
You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
add a comment |
$begingroup$
Your made the sign error in step 2 (as indicated by KaviRamaMurthy).
As an alternative, you can simplify in step 2 before integrating:
$$int_0^{0.5}left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx=
int_0^{0.5}left(require{cancel}cancel{frac{x}{2}} -x^2+frac{1}{8}-cancel{frac x2}+frac{x^2}{2}right) dx=\
int_0^{0.5}left(-frac{x^2}{2}+frac18right) dx=
left(-frac{x^3}{6}+frac x8right)|_0^{0.5}=
-frac{1}{48}+frac{1}{16}=
frac 1{24}.$$
Answering your comment, yes, for the upper limit $0.5$ the term will be $0$, but for the lower limit $0$, the term will be $frac1{48}$:
$$(frac{x^2}{4} - frac{x^3}{3} color{red}{-} frac{(0.5-x)^3}{6})Big|_0^{0.5}=left(frac{1}{16}-frac1{24}-0right)-left(0-0-frac1{48}right)=frac1{24}.$$
$endgroup$
$begingroup$
Farruhota, thank you for the awesome feedback. I really appreciate your help.
$endgroup$
– MitterHai
Dec 5 '18 at 11:42
$begingroup$
You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
add a comment |
$begingroup$
Your made the sign error in step 2 (as indicated by KaviRamaMurthy).
As an alternative, you can simplify in step 2 before integrating:
$$int_0^{0.5}left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx=
int_0^{0.5}left(require{cancel}cancel{frac{x}{2}} -x^2+frac{1}{8}-cancel{frac x2}+frac{x^2}{2}right) dx=\
int_0^{0.5}left(-frac{x^2}{2}+frac18right) dx=
left(-frac{x^3}{6}+frac x8right)|_0^{0.5}=
-frac{1}{48}+frac{1}{16}=
frac 1{24}.$$
Answering your comment, yes, for the upper limit $0.5$ the term will be $0$, but for the lower limit $0$, the term will be $frac1{48}$:
$$(frac{x^2}{4} - frac{x^3}{3} color{red}{-} frac{(0.5-x)^3}{6})Big|_0^{0.5}=left(frac{1}{16}-frac1{24}-0right)-left(0-0-frac1{48}right)=frac1{24}.$$
$endgroup$
Your made the sign error in step 2 (as indicated by KaviRamaMurthy).
As an alternative, you can simplify in step 2 before integrating:
$$int_0^{0.5}left(frac{x}{2} -x^2+frac{(0.5-x)^2}{2}right) dx=
int_0^{0.5}left(require{cancel}cancel{frac{x}{2}} -x^2+frac{1}{8}-cancel{frac x2}+frac{x^2}{2}right) dx=\
int_0^{0.5}left(-frac{x^2}{2}+frac18right) dx=
left(-frac{x^3}{6}+frac x8right)|_0^{0.5}=
-frac{1}{48}+frac{1}{16}=
frac 1{24}.$$
Answering your comment, yes, for the upper limit $0.5$ the term will be $0$, but for the lower limit $0$, the term will be $frac1{48}$:
$$(frac{x^2}{4} - frac{x^3}{3} color{red}{-} frac{(0.5-x)^3}{6})Big|_0^{0.5}=left(frac{1}{16}-frac1{24}-0right)-left(0-0-frac1{48}right)=frac1{24}.$$
answered Dec 5 '18 at 11:22
farruhotafarruhota
20.2k2738
20.2k2738
$begingroup$
Farruhota, thank you for the awesome feedback. I really appreciate your help.
$endgroup$
– MitterHai
Dec 5 '18 at 11:42
$begingroup$
You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
add a comment |
$begingroup$
Farruhota, thank you for the awesome feedback. I really appreciate your help.
$endgroup$
– MitterHai
Dec 5 '18 at 11:42
$begingroup$
You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
$begingroup$
Farruhota, thank you for the awesome feedback. I really appreciate your help.
$endgroup$
– MitterHai
Dec 5 '18 at 11:42
$begingroup$
Farruhota, thank you for the awesome feedback. I really appreciate your help.
$endgroup$
– MitterHai
Dec 5 '18 at 11:42
$begingroup$
You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
$begingroup$
You are welcome. Good luck.
$endgroup$
– farruhota
Dec 5 '18 at 11:43
add a comment |
$begingroup$
Anti-derivative of $frac {(0.5-x)^{2}} 2$ is $-frac {(0.5-x)^{3}} 6$. You are missing the minus sign.
$endgroup$
$begingroup$
Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
$endgroup$
– MitterHai
Dec 5 '18 at 10:38
$begingroup$
@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
$begingroup$
What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
add a comment |
$begingroup$
Anti-derivative of $frac {(0.5-x)^{2}} 2$ is $-frac {(0.5-x)^{3}} 6$. You are missing the minus sign.
$endgroup$
$begingroup$
Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
$endgroup$
– MitterHai
Dec 5 '18 at 10:38
$begingroup$
@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
$begingroup$
What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
add a comment |
$begingroup$
Anti-derivative of $frac {(0.5-x)^{2}} 2$ is $-frac {(0.5-x)^{3}} 6$. You are missing the minus sign.
$endgroup$
Anti-derivative of $frac {(0.5-x)^{2}} 2$ is $-frac {(0.5-x)^{3}} 6$. You are missing the minus sign.
answered Dec 5 '18 at 10:22
Kavi Rama MurthyKavi Rama Murthy
57.7k42160
57.7k42160
$begingroup$
Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
$endgroup$
– MitterHai
Dec 5 '18 at 10:38
$begingroup$
@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
$begingroup$
What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
add a comment |
$begingroup$
Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
$endgroup$
– MitterHai
Dec 5 '18 at 10:38
$begingroup$
@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
$begingroup$
What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
$begingroup$
Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
$endgroup$
– MitterHai
Dec 5 '18 at 10:38
$begingroup$
Hi Kavi, thank you for taking on this problem, I figured out how the anti-derivative should have a minus sign, but when you evaluate it at 0.5, doesn't this part of the equation just go to 0?
$endgroup$
– MitterHai
Dec 5 '18 at 10:38
$begingroup$
@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
$begingroup$
@MitterHai When you evaluate $-(0.5 - x)^3/6$ at $0$, you get a non-zero value. This fixes the problem.
$endgroup$
– littleO
Dec 5 '18 at 11:22
$begingroup$
What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
$begingroup$
What a face palm moment. Thanks Pro..fe.. I mean littleO!
$endgroup$
– MitterHai
Dec 5 '18 at 11:40
add a comment |
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