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The proportionality of $d^2y/dx^2$ and $y$



The 2019 Stack Overflow Developer Survey Results Are InIntegral $int_0^infty log(1+x^2)fraccoshfracpi x2sinh^2fracpi x2mathrm dx=2-frac4pi$Prove second derivative of $g$ is proportional to $g^2$Evaluating $int_0^2 fracdxsqrt[3]2x^2-x^3$Second order differential equation falling boxlaplace transform of smirnov density, i.e. how to calculate this integral?Showing the proportionality? (Calculus)Chemistry using integrationDifferential Equation.Proportionality, integrationcalculating $int_0 ^3 xsqrtdx$










1












$begingroup$



Suppose that $x$ and $y$ are related by the equation $displaystyle x=int_0^yfrac1sqrt1+4t^2,dt$. Show that $d^2y/dx^2$ is proportional to $y$ and find the constant of proportionality.




I try to calculate the problem and I got the proportionality is $4(1+4y^2)^-1/2$
But it is not a constant.



Thank you so much.










share|cite|improve this question











$endgroup$











  • $begingroup$
    How do you get your result?
    $endgroup$
    – Oscar Lanzi
    Mar 30 at 12:03






  • 1




    $begingroup$
    I seen that I forgot the dy/dx at the end,thank you.
    $endgroup$
    – user630900
    Mar 30 at 12:22















1












$begingroup$



Suppose that $x$ and $y$ are related by the equation $displaystyle x=int_0^yfrac1sqrt1+4t^2,dt$. Show that $d^2y/dx^2$ is proportional to $y$ and find the constant of proportionality.




I try to calculate the problem and I got the proportionality is $4(1+4y^2)^-1/2$
But it is not a constant.



Thank you so much.










share|cite|improve this question











$endgroup$











  • $begingroup$
    How do you get your result?
    $endgroup$
    – Oscar Lanzi
    Mar 30 at 12:03






  • 1




    $begingroup$
    I seen that I forgot the dy/dx at the end,thank you.
    $endgroup$
    – user630900
    Mar 30 at 12:22













1












1








1





$begingroup$



Suppose that $x$ and $y$ are related by the equation $displaystyle x=int_0^yfrac1sqrt1+4t^2,dt$. Show that $d^2y/dx^2$ is proportional to $y$ and find the constant of proportionality.




I try to calculate the problem and I got the proportionality is $4(1+4y^2)^-1/2$
But it is not a constant.



Thank you so much.










share|cite|improve this question











$endgroup$





Suppose that $x$ and $y$ are related by the equation $displaystyle x=int_0^yfrac1sqrt1+4t^2,dt$. Show that $d^2y/dx^2$ is proportional to $y$ and find the constant of proportionality.




I try to calculate the problem and I got the proportionality is $4(1+4y^2)^-1/2$
But it is not a constant.



Thank you so much.







calculus






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Mar 30 at 12:08









egreg

185k1486208




185k1486208










asked Mar 30 at 11:47









user630900user630900

12




12











  • $begingroup$
    How do you get your result?
    $endgroup$
    – Oscar Lanzi
    Mar 30 at 12:03






  • 1




    $begingroup$
    I seen that I forgot the dy/dx at the end,thank you.
    $endgroup$
    – user630900
    Mar 30 at 12:22
















  • $begingroup$
    How do you get your result?
    $endgroup$
    – Oscar Lanzi
    Mar 30 at 12:03






  • 1




    $begingroup$
    I seen that I forgot the dy/dx at the end,thank you.
    $endgroup$
    – user630900
    Mar 30 at 12:22















$begingroup$
How do you get your result?
$endgroup$
– Oscar Lanzi
Mar 30 at 12:03




$begingroup$
How do you get your result?
$endgroup$
– Oscar Lanzi
Mar 30 at 12:03




1




1




$begingroup$
I seen that I forgot the dy/dx at the end,thank you.
$endgroup$
– user630900
Mar 30 at 12:22




$begingroup$
I seen that I forgot the dy/dx at the end,thank you.
$endgroup$
– user630900
Mar 30 at 12:22










3 Answers
3






active

oldest

votes


















2












$begingroup$

Here is a solution that gives explicitly the expression of functions we are working on.



You may know that



$$textthe antiderivative of dfrac1sqrt1+t^2 textis x=sinh^-1(t)+k$$



where $k$ is an arbitrary constant (sinh$^-1$ being the inverse hyperbolic sine also denoted arcsinh).



Thus, using an elementary change of variable,



$$textthe antiderivative of dfrac1sqrt1+(2y)^2 is x=tfrac12 textsinh^-1(2y)+k tag1$$



As $x$ is expressed in terms of $y$, we are invited to reverse (1) for expressing $y$ as a function of $x$ ; doing this is easy : first, transform (1) into :



$$2(x-k)=textsinh^-1(2y)$$



then take the $sinh$ of LHS and RHS ; we get



$$sinh(2(x-k))=2y iff $$




$$y=tfrac12 sinh(2(x-k))tag2$$




From there, you can easily check that the second derivative of $y$ is $4$ times $y$.



Remark : Of course, the converse is not true. (2) is not, by far, the most general solution to the different and classical issue :




Find all functions proportional to their second derivative,




which has to be divided into two cases, according to the sign of this proportionality constant (of the form $k^2$ or $-k^2$) yielding two second order ordinary differential equations (ODE) :



$$fracd^2ydt^2=k^2 y textand fracd^2ydt^2=-k^2 y tag3$$



with resp. solutions :



$$y=A textsinh(kx+B) textand y=A sin(kx+B)$$



where $A$ and $B$ are arbitrary constants.



(The second ODE in (3) is called the ODE of harmonic oscillator).






share|cite|improve this answer











$endgroup$












  • $begingroup$
    Thank you for your answer.
    $endgroup$
    – user630900
    Mar 30 at 16:43


















1












$begingroup$

$$fracdydx=left(fracdxdyright)^-1=left(fracddyint_0^yfrac1sqrt1+4t^2right)^-1=sqrt1+4y^2$$
so
$$fracd^2ydx^2=fracddxsqrt1+4y^2=frac4yfracdydxsqrt1+4y^2=4y$$
the proportionality is $4$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23


















1












$begingroup$

The constant is $4$: $frac dx dy =(1+4y^2)^-1/2$ so $frac dy dx =(1+4y^2)^1/2$. This gives $frac d^2y dx^2 =4y (1+4y^2)^-1/2frac dy dx=4y$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23











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3 Answers
3






active

oldest

votes








3 Answers
3






active

oldest

votes









active

oldest

votes






active

oldest

votes









2












$begingroup$

Here is a solution that gives explicitly the expression of functions we are working on.



You may know that



$$textthe antiderivative of dfrac1sqrt1+t^2 textis x=sinh^-1(t)+k$$



where $k$ is an arbitrary constant (sinh$^-1$ being the inverse hyperbolic sine also denoted arcsinh).



Thus, using an elementary change of variable,



$$textthe antiderivative of dfrac1sqrt1+(2y)^2 is x=tfrac12 textsinh^-1(2y)+k tag1$$



As $x$ is expressed in terms of $y$, we are invited to reverse (1) for expressing $y$ as a function of $x$ ; doing this is easy : first, transform (1) into :



$$2(x-k)=textsinh^-1(2y)$$



then take the $sinh$ of LHS and RHS ; we get



$$sinh(2(x-k))=2y iff $$




$$y=tfrac12 sinh(2(x-k))tag2$$




From there, you can easily check that the second derivative of $y$ is $4$ times $y$.



Remark : Of course, the converse is not true. (2) is not, by far, the most general solution to the different and classical issue :




Find all functions proportional to their second derivative,




which has to be divided into two cases, according to the sign of this proportionality constant (of the form $k^2$ or $-k^2$) yielding two second order ordinary differential equations (ODE) :



$$fracd^2ydt^2=k^2 y textand fracd^2ydt^2=-k^2 y tag3$$



with resp. solutions :



$$y=A textsinh(kx+B) textand y=A sin(kx+B)$$



where $A$ and $B$ are arbitrary constants.



(The second ODE in (3) is called the ODE of harmonic oscillator).






share|cite|improve this answer











$endgroup$












  • $begingroup$
    Thank you for your answer.
    $endgroup$
    – user630900
    Mar 30 at 16:43















2












$begingroup$

Here is a solution that gives explicitly the expression of functions we are working on.



You may know that



$$textthe antiderivative of dfrac1sqrt1+t^2 textis x=sinh^-1(t)+k$$



where $k$ is an arbitrary constant (sinh$^-1$ being the inverse hyperbolic sine also denoted arcsinh).



Thus, using an elementary change of variable,



$$textthe antiderivative of dfrac1sqrt1+(2y)^2 is x=tfrac12 textsinh^-1(2y)+k tag1$$



As $x$ is expressed in terms of $y$, we are invited to reverse (1) for expressing $y$ as a function of $x$ ; doing this is easy : first, transform (1) into :



$$2(x-k)=textsinh^-1(2y)$$



then take the $sinh$ of LHS and RHS ; we get



$$sinh(2(x-k))=2y iff $$




$$y=tfrac12 sinh(2(x-k))tag2$$




From there, you can easily check that the second derivative of $y$ is $4$ times $y$.



Remark : Of course, the converse is not true. (2) is not, by far, the most general solution to the different and classical issue :




Find all functions proportional to their second derivative,




which has to be divided into two cases, according to the sign of this proportionality constant (of the form $k^2$ or $-k^2$) yielding two second order ordinary differential equations (ODE) :



$$fracd^2ydt^2=k^2 y textand fracd^2ydt^2=-k^2 y tag3$$



with resp. solutions :



$$y=A textsinh(kx+B) textand y=A sin(kx+B)$$



where $A$ and $B$ are arbitrary constants.



(The second ODE in (3) is called the ODE of harmonic oscillator).






share|cite|improve this answer











$endgroup$












  • $begingroup$
    Thank you for your answer.
    $endgroup$
    – user630900
    Mar 30 at 16:43













2












2








2





$begingroup$

Here is a solution that gives explicitly the expression of functions we are working on.



You may know that



$$textthe antiderivative of dfrac1sqrt1+t^2 textis x=sinh^-1(t)+k$$



where $k$ is an arbitrary constant (sinh$^-1$ being the inverse hyperbolic sine also denoted arcsinh).



Thus, using an elementary change of variable,



$$textthe antiderivative of dfrac1sqrt1+(2y)^2 is x=tfrac12 textsinh^-1(2y)+k tag1$$



As $x$ is expressed in terms of $y$, we are invited to reverse (1) for expressing $y$ as a function of $x$ ; doing this is easy : first, transform (1) into :



$$2(x-k)=textsinh^-1(2y)$$



then take the $sinh$ of LHS and RHS ; we get



$$sinh(2(x-k))=2y iff $$




$$y=tfrac12 sinh(2(x-k))tag2$$




From there, you can easily check that the second derivative of $y$ is $4$ times $y$.



Remark : Of course, the converse is not true. (2) is not, by far, the most general solution to the different and classical issue :




Find all functions proportional to their second derivative,




which has to be divided into two cases, according to the sign of this proportionality constant (of the form $k^2$ or $-k^2$) yielding two second order ordinary differential equations (ODE) :



$$fracd^2ydt^2=k^2 y textand fracd^2ydt^2=-k^2 y tag3$$



with resp. solutions :



$$y=A textsinh(kx+B) textand y=A sin(kx+B)$$



where $A$ and $B$ are arbitrary constants.



(The second ODE in (3) is called the ODE of harmonic oscillator).






share|cite|improve this answer











$endgroup$



Here is a solution that gives explicitly the expression of functions we are working on.



You may know that



$$textthe antiderivative of dfrac1sqrt1+t^2 textis x=sinh^-1(t)+k$$



where $k$ is an arbitrary constant (sinh$^-1$ being the inverse hyperbolic sine also denoted arcsinh).



Thus, using an elementary change of variable,



$$textthe antiderivative of dfrac1sqrt1+(2y)^2 is x=tfrac12 textsinh^-1(2y)+k tag1$$



As $x$ is expressed in terms of $y$, we are invited to reverse (1) for expressing $y$ as a function of $x$ ; doing this is easy : first, transform (1) into :



$$2(x-k)=textsinh^-1(2y)$$



then take the $sinh$ of LHS and RHS ; we get



$$sinh(2(x-k))=2y iff $$




$$y=tfrac12 sinh(2(x-k))tag2$$




From there, you can easily check that the second derivative of $y$ is $4$ times $y$.



Remark : Of course, the converse is not true. (2) is not, by far, the most general solution to the different and classical issue :




Find all functions proportional to their second derivative,




which has to be divided into two cases, according to the sign of this proportionality constant (of the form $k^2$ or $-k^2$) yielding two second order ordinary differential equations (ODE) :



$$fracd^2ydt^2=k^2 y textand fracd^2ydt^2=-k^2 y tag3$$



with resp. solutions :



$$y=A textsinh(kx+B) textand y=A sin(kx+B)$$



where $A$ and $B$ are arbitrary constants.



(The second ODE in (3) is called the ODE of harmonic oscillator).







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Mar 31 at 7:39

























answered Mar 30 at 16:06









Jean MarieJean Marie

31.5k42355




31.5k42355











  • $begingroup$
    Thank you for your answer.
    $endgroup$
    – user630900
    Mar 30 at 16:43
















  • $begingroup$
    Thank you for your answer.
    $endgroup$
    – user630900
    Mar 30 at 16:43















$begingroup$
Thank you for your answer.
$endgroup$
– user630900
Mar 30 at 16:43




$begingroup$
Thank you for your answer.
$endgroup$
– user630900
Mar 30 at 16:43











1












$begingroup$

$$fracdydx=left(fracdxdyright)^-1=left(fracddyint_0^yfrac1sqrt1+4t^2right)^-1=sqrt1+4y^2$$
so
$$fracd^2ydx^2=fracddxsqrt1+4y^2=frac4yfracdydxsqrt1+4y^2=4y$$
the proportionality is $4$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23















1












$begingroup$

$$fracdydx=left(fracdxdyright)^-1=left(fracddyint_0^yfrac1sqrt1+4t^2right)^-1=sqrt1+4y^2$$
so
$$fracd^2ydx^2=fracddxsqrt1+4y^2=frac4yfracdydxsqrt1+4y^2=4y$$
the proportionality is $4$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23













1












1








1





$begingroup$

$$fracdydx=left(fracdxdyright)^-1=left(fracddyint_0^yfrac1sqrt1+4t^2right)^-1=sqrt1+4y^2$$
so
$$fracd^2ydx^2=fracddxsqrt1+4y^2=frac4yfracdydxsqrt1+4y^2=4y$$
the proportionality is $4$.






share|cite|improve this answer









$endgroup$



$$fracdydx=left(fracdxdyright)^-1=left(fracddyint_0^yfrac1sqrt1+4t^2right)^-1=sqrt1+4y^2$$
so
$$fracd^2ydx^2=fracddxsqrt1+4y^2=frac4yfracdydxsqrt1+4y^2=4y$$
the proportionality is $4$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Mar 30 at 12:07









Xin FuXin Fu

34319




34319











  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23
















  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23















$begingroup$
Thank you very much.
$endgroup$
– user630900
Mar 30 at 12:23




$begingroup$
Thank you very much.
$endgroup$
– user630900
Mar 30 at 12:23











1












$begingroup$

The constant is $4$: $frac dx dy =(1+4y^2)^-1/2$ so $frac dy dx =(1+4y^2)^1/2$. This gives $frac d^2y dx^2 =4y (1+4y^2)^-1/2frac dy dx=4y$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23















1












$begingroup$

The constant is $4$: $frac dx dy =(1+4y^2)^-1/2$ so $frac dy dx =(1+4y^2)^1/2$. This gives $frac d^2y dx^2 =4y (1+4y^2)^-1/2frac dy dx=4y$.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23













1












1








1





$begingroup$

The constant is $4$: $frac dx dy =(1+4y^2)^-1/2$ so $frac dy dx =(1+4y^2)^1/2$. This gives $frac d^2y dx^2 =4y (1+4y^2)^-1/2frac dy dx=4y$.






share|cite|improve this answer









$endgroup$



The constant is $4$: $frac dx dy =(1+4y^2)^-1/2$ so $frac dy dx =(1+4y^2)^1/2$. This gives $frac d^2y dx^2 =4y (1+4y^2)^-1/2frac dy dx=4y$.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Mar 30 at 12:12









Kavi Rama MurthyKavi Rama Murthy

73.6k53170




73.6k53170











  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23
















  • $begingroup$
    Thank you very much.
    $endgroup$
    – user630900
    Mar 30 at 12:23















$begingroup$
Thank you very much.
$endgroup$
– user630900
Mar 30 at 12:23




$begingroup$
Thank you very much.
$endgroup$
– user630900
Mar 30 at 12:23

















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