Finding the function from a definite integral?

Multi tool use
$begingroup$
If I had an integral, as shown below,
$$dfrac{int_{0}^{1} a(x) cdot x^{2} ; dx}{int_{0}^{1} a(x); dx}=dfrac{1}{2}$$
How would I figure out the function $a(x)$, is there way or do I have to make a guess and test it?
calculus
$endgroup$
add a comment |
$begingroup$
If I had an integral, as shown below,
$$dfrac{int_{0}^{1} a(x) cdot x^{2} ; dx}{int_{0}^{1} a(x); dx}=dfrac{1}{2}$$
How would I figure out the function $a(x)$, is there way or do I have to make a guess and test it?
calculus
$endgroup$
1
$begingroup$
The function is simply not fixed. This is more or less like asking given $f(1)=2019$, what is $f(x)$?
$endgroup$
– Jack D'Aurizio
Jan 16 at 12:39
1
$begingroup$
Every non-zero constant function will do, also $a(x) = x$ from polynomials
$endgroup$
– Jakobian
Jan 16 at 12:49
1
$begingroup$
@Jakobian $a(x)=c$ will give $frac{frac c3}{c}=frac13$.
$endgroup$
– W. Zhu
Jan 16 at 12:55
add a comment |
$begingroup$
If I had an integral, as shown below,
$$dfrac{int_{0}^{1} a(x) cdot x^{2} ; dx}{int_{0}^{1} a(x); dx}=dfrac{1}{2}$$
How would I figure out the function $a(x)$, is there way or do I have to make a guess and test it?
calculus
$endgroup$
If I had an integral, as shown below,
$$dfrac{int_{0}^{1} a(x) cdot x^{2} ; dx}{int_{0}^{1} a(x); dx}=dfrac{1}{2}$$
How would I figure out the function $a(x)$, is there way or do I have to make a guess and test it?
calculus
calculus
asked Jan 16 at 12:37
D. QaD. Qa
1656
1656
1
$begingroup$
The function is simply not fixed. This is more or less like asking given $f(1)=2019$, what is $f(x)$?
$endgroup$
– Jack D'Aurizio
Jan 16 at 12:39
1
$begingroup$
Every non-zero constant function will do, also $a(x) = x$ from polynomials
$endgroup$
– Jakobian
Jan 16 at 12:49
1
$begingroup$
@Jakobian $a(x)=c$ will give $frac{frac c3}{c}=frac13$.
$endgroup$
– W. Zhu
Jan 16 at 12:55
add a comment |
1
$begingroup$
The function is simply not fixed. This is more or less like asking given $f(1)=2019$, what is $f(x)$?
$endgroup$
– Jack D'Aurizio
Jan 16 at 12:39
1
$begingroup$
Every non-zero constant function will do, also $a(x) = x$ from polynomials
$endgroup$
– Jakobian
Jan 16 at 12:49
1
$begingroup$
@Jakobian $a(x)=c$ will give $frac{frac c3}{c}=frac13$.
$endgroup$
– W. Zhu
Jan 16 at 12:55
1
1
$begingroup$
The function is simply not fixed. This is more or less like asking given $f(1)=2019$, what is $f(x)$?
$endgroup$
– Jack D'Aurizio
Jan 16 at 12:39
$begingroup$
The function is simply not fixed. This is more or less like asking given $f(1)=2019$, what is $f(x)$?
$endgroup$
– Jack D'Aurizio
Jan 16 at 12:39
1
1
$begingroup$
Every non-zero constant function will do, also $a(x) = x$ from polynomials
$endgroup$
– Jakobian
Jan 16 at 12:49
$begingroup$
Every non-zero constant function will do, also $a(x) = x$ from polynomials
$endgroup$
– Jakobian
Jan 16 at 12:49
1
1
$begingroup$
@Jakobian $a(x)=c$ will give $frac{frac c3}{c}=frac13$.
$endgroup$
– W. Zhu
Jan 16 at 12:55
$begingroup$
@Jakobian $a(x)=c$ will give $frac{frac c3}{c}=frac13$.
$endgroup$
– W. Zhu
Jan 16 at 12:55
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
There are too many choices. At some point you're going to have to make a choice arbitrarily, or use information from the deeper context of the question (things you haven't told us about) to choose.
We can see $a(x) = 1$ (or any constant) won't work. The average value of $x^2$ for $x in [0,1]$ is only $frac13.$ We need to "weight the average" to count the larger values of $x^2$ more heavily so that the "average" comes out to $frac12$ instead.
The function $a(x)$ is the "weight".
One possible approach is to assume $a(x) = x^k$ for some real number $k$. Work out the ratio of the integrals, which will be some expression in $k$.
You want that expression to equal $frac12.$ Assume it does, and solve for $k.$
There are many other possibilities, for example if you don't mind an integrand with finitely many jump discontinuities then $a(x) = lfloor x + krfloor$ could do the job.
$endgroup$
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
1
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
add a comment |
$begingroup$
One possibility is the function $a(x)=0$ for all $x$ but there might be others.
$endgroup$
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
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active
oldest
votes
$begingroup$
There are too many choices. At some point you're going to have to make a choice arbitrarily, or use information from the deeper context of the question (things you haven't told us about) to choose.
We can see $a(x) = 1$ (or any constant) won't work. The average value of $x^2$ for $x in [0,1]$ is only $frac13.$ We need to "weight the average" to count the larger values of $x^2$ more heavily so that the "average" comes out to $frac12$ instead.
The function $a(x)$ is the "weight".
One possible approach is to assume $a(x) = x^k$ for some real number $k$. Work out the ratio of the integrals, which will be some expression in $k$.
You want that expression to equal $frac12.$ Assume it does, and solve for $k.$
There are many other possibilities, for example if you don't mind an integrand with finitely many jump discontinuities then $a(x) = lfloor x + krfloor$ could do the job.
$endgroup$
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
1
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
add a comment |
$begingroup$
There are too many choices. At some point you're going to have to make a choice arbitrarily, or use information from the deeper context of the question (things you haven't told us about) to choose.
We can see $a(x) = 1$ (or any constant) won't work. The average value of $x^2$ for $x in [0,1]$ is only $frac13.$ We need to "weight the average" to count the larger values of $x^2$ more heavily so that the "average" comes out to $frac12$ instead.
The function $a(x)$ is the "weight".
One possible approach is to assume $a(x) = x^k$ for some real number $k$. Work out the ratio of the integrals, which will be some expression in $k$.
You want that expression to equal $frac12.$ Assume it does, and solve for $k.$
There are many other possibilities, for example if you don't mind an integrand with finitely many jump discontinuities then $a(x) = lfloor x + krfloor$ could do the job.
$endgroup$
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
1
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
add a comment |
$begingroup$
There are too many choices. At some point you're going to have to make a choice arbitrarily, or use information from the deeper context of the question (things you haven't told us about) to choose.
We can see $a(x) = 1$ (or any constant) won't work. The average value of $x^2$ for $x in [0,1]$ is only $frac13.$ We need to "weight the average" to count the larger values of $x^2$ more heavily so that the "average" comes out to $frac12$ instead.
The function $a(x)$ is the "weight".
One possible approach is to assume $a(x) = x^k$ for some real number $k$. Work out the ratio of the integrals, which will be some expression in $k$.
You want that expression to equal $frac12.$ Assume it does, and solve for $k.$
There are many other possibilities, for example if you don't mind an integrand with finitely many jump discontinuities then $a(x) = lfloor x + krfloor$ could do the job.
$endgroup$
There are too many choices. At some point you're going to have to make a choice arbitrarily, or use information from the deeper context of the question (things you haven't told us about) to choose.
We can see $a(x) = 1$ (or any constant) won't work. The average value of $x^2$ for $x in [0,1]$ is only $frac13.$ We need to "weight the average" to count the larger values of $x^2$ more heavily so that the "average" comes out to $frac12$ instead.
The function $a(x)$ is the "weight".
One possible approach is to assume $a(x) = x^k$ for some real number $k$. Work out the ratio of the integrals, which will be some expression in $k$.
You want that expression to equal $frac12.$ Assume it does, and solve for $k.$
There are many other possibilities, for example if you don't mind an integrand with finitely many jump discontinuities then $a(x) = lfloor x + krfloor$ could do the job.
answered Jan 16 at 12:58
David KDavid K
55.5k345121
55.5k345121
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
1
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
add a comment |
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
1
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
$begingroup$
I did assume $x^{k}$ and after solving, managed to get a value for $k$, which is $k=1$, therefore $a(x)=x$. That was very helpful, thank you so much!
$endgroup$
– D. Qa
Jan 16 at 13:26
1
1
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
The solution $a(x) = x$ was also given away in a comment under the question. But it seems more satisfying to derive it from a method that doesn't assume you have guessed the exact answer immediately.
$endgroup$
– David K
Jan 16 at 13:33
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
$begingroup$
a(x) can be x but as pointed out, there are lots of other possibilities too.
$endgroup$
– Paul
Jan 16 at 14:03
add a comment |
$begingroup$
One possibility is the function $a(x)=0$ for all $x$ but there might be others.
$endgroup$
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
add a comment |
$begingroup$
One possibility is the function $a(x)=0$ for all $x$ but there might be others.
$endgroup$
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
add a comment |
$begingroup$
One possibility is the function $a(x)=0$ for all $x$ but there might be others.
$endgroup$
One possibility is the function $a(x)=0$ for all $x$ but there might be others.
answered Jan 16 at 12:43
JamesJames
2,112422
2,112422
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
add a comment |
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
But then we divide by $0$
$endgroup$
– Jakobian
Jan 16 at 12:43
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
You can rewrite the formula by bringing the denominator on the right site. Then there are no issues with dividing by 0 and the solution is the same.
$endgroup$
– James
Jan 16 at 12:44
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
$frac00$ is indeterminate. You can make $frac00=1$ and $frac00=2$ by "moving the denominator to the other side", but $1ne2$.
$endgroup$
– W. Zhu
Jan 16 at 13:03
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
$begingroup$
I know. But the equation in question is $int dots = frac{1}{2}int dots$ which can be solved without any problems.
$endgroup$
– James
Jan 16 at 13:06
add a comment |
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F4jCIrzsxyveOi0URKsuwzBY,2UqADQVAcFroDTCK219v7Ol nEZMkLLk,WYk8yBBJQdBX,DM8f
1
$begingroup$
The function is simply not fixed. This is more or less like asking given $f(1)=2019$, what is $f(x)$?
$endgroup$
– Jack D'Aurizio
Jan 16 at 12:39
1
$begingroup$
Every non-zero constant function will do, also $a(x) = x$ from polynomials
$endgroup$
– Jakobian
Jan 16 at 12:49
1
$begingroup$
@Jakobian $a(x)=c$ will give $frac{frac c3}{c}=frac13$.
$endgroup$
– W. Zhu
Jan 16 at 12:55