$y$ be the solution of $y'+y=|x|,xin mathbb{R}$, find $y(1)$ [duplicate]
$begingroup$
This question already has an answer here:
Find the value of $y(1)$ of the ODE $y'+y=|x|$.
3 answers
Let $y$ be the solution of $$y'+y=|x|,xin mathbb{R}$$
$$y(-1)=0$$
Then what is the value of $y(1)$:
Efforts:
I know how to solve the linear equations of general form $$y'+Py=Q$$ where $P$ and $Q$ are continuous function of $x$.
In my case $P=1$ and $Q=|x|$ and both are continuous.
Now if I apply the formula I get
$$ye^x=int e^x |x|dx+C$$
Now if I put the initial value I get $$0=int e^{-1}dx+C$$
Now what should I do? I mean what should be the range of integration and how I proceed from here.
Thanks.
ordinary-differential-equations
$endgroup$
marked as duplicate by StammeringMathematician, Community♦ Jan 31 at 3:29
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
add a comment |
$begingroup$
This question already has an answer here:
Find the value of $y(1)$ of the ODE $y'+y=|x|$.
3 answers
Let $y$ be the solution of $$y'+y=|x|,xin mathbb{R}$$
$$y(-1)=0$$
Then what is the value of $y(1)$:
Efforts:
I know how to solve the linear equations of general form $$y'+Py=Q$$ where $P$ and $Q$ are continuous function of $x$.
In my case $P=1$ and $Q=|x|$ and both are continuous.
Now if I apply the formula I get
$$ye^x=int e^x |x|dx+C$$
Now if I put the initial value I get $$0=int e^{-1}dx+C$$
Now what should I do? I mean what should be the range of integration and how I proceed from here.
Thanks.
ordinary-differential-equations
$endgroup$
marked as duplicate by StammeringMathematician, Community♦ Jan 31 at 3:29
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
$begingroup$
I have voted my question as duplicate. I have found the solution here math.stackexchange.com/questions/1850959/… I am not deleting it as it has received two upvotes and one favorites. Thanks
$endgroup$
– StammeringMathematician
Jan 31 at 3:28
1
$begingroup$
"Now if I put the initial value I get" What? Come again? What do you think the formula just after that even means?
$endgroup$
– Did
Jan 31 at 7:22
$begingroup$
@Did Can you explain? I did not get it
$endgroup$
– StammeringMathematician
Jan 31 at 10:12
1
$begingroup$
Again, what do you mean by "Now if I put the initial value I get $0=int e^{-1}dx+C$"? Can you explain? I do not get it.
$endgroup$
– Did
Jan 31 at 10:56
add a comment |
$begingroup$
This question already has an answer here:
Find the value of $y(1)$ of the ODE $y'+y=|x|$.
3 answers
Let $y$ be the solution of $$y'+y=|x|,xin mathbb{R}$$
$$y(-1)=0$$
Then what is the value of $y(1)$:
Efforts:
I know how to solve the linear equations of general form $$y'+Py=Q$$ where $P$ and $Q$ are continuous function of $x$.
In my case $P=1$ and $Q=|x|$ and both are continuous.
Now if I apply the formula I get
$$ye^x=int e^x |x|dx+C$$
Now if I put the initial value I get $$0=int e^{-1}dx+C$$
Now what should I do? I mean what should be the range of integration and how I proceed from here.
Thanks.
ordinary-differential-equations
$endgroup$
This question already has an answer here:
Find the value of $y(1)$ of the ODE $y'+y=|x|$.
3 answers
Let $y$ be the solution of $$y'+y=|x|,xin mathbb{R}$$
$$y(-1)=0$$
Then what is the value of $y(1)$:
Efforts:
I know how to solve the linear equations of general form $$y'+Py=Q$$ where $P$ and $Q$ are continuous function of $x$.
In my case $P=1$ and $Q=|x|$ and both are continuous.
Now if I apply the formula I get
$$ye^x=int e^x |x|dx+C$$
Now if I put the initial value I get $$0=int e^{-1}dx+C$$
Now what should I do? I mean what should be the range of integration and how I proceed from here.
Thanks.
This question already has an answer here:
Find the value of $y(1)$ of the ODE $y'+y=|x|$.
3 answers
ordinary-differential-equations
ordinary-differential-equations
asked Jan 31 at 1:27


StammeringMathematicianStammeringMathematician
2,7311324
2,7311324
marked as duplicate by StammeringMathematician, Community♦ Jan 31 at 3:29
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
marked as duplicate by StammeringMathematician, Community♦ Jan 31 at 3:29
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
$begingroup$
I have voted my question as duplicate. I have found the solution here math.stackexchange.com/questions/1850959/… I am not deleting it as it has received two upvotes and one favorites. Thanks
$endgroup$
– StammeringMathematician
Jan 31 at 3:28
1
$begingroup$
"Now if I put the initial value I get" What? Come again? What do you think the formula just after that even means?
$endgroup$
– Did
Jan 31 at 7:22
$begingroup$
@Did Can you explain? I did not get it
$endgroup$
– StammeringMathematician
Jan 31 at 10:12
1
$begingroup$
Again, what do you mean by "Now if I put the initial value I get $0=int e^{-1}dx+C$"? Can you explain? I do not get it.
$endgroup$
– Did
Jan 31 at 10:56
add a comment |
$begingroup$
I have voted my question as duplicate. I have found the solution here math.stackexchange.com/questions/1850959/… I am not deleting it as it has received two upvotes and one favorites. Thanks
$endgroup$
– StammeringMathematician
Jan 31 at 3:28
1
$begingroup$
"Now if I put the initial value I get" What? Come again? What do you think the formula just after that even means?
$endgroup$
– Did
Jan 31 at 7:22
$begingroup$
@Did Can you explain? I did not get it
$endgroup$
– StammeringMathematician
Jan 31 at 10:12
1
$begingroup$
Again, what do you mean by "Now if I put the initial value I get $0=int e^{-1}dx+C$"? Can you explain? I do not get it.
$endgroup$
– Did
Jan 31 at 10:56
$begingroup$
I have voted my question as duplicate. I have found the solution here math.stackexchange.com/questions/1850959/… I am not deleting it as it has received two upvotes and one favorites. Thanks
$endgroup$
– StammeringMathematician
Jan 31 at 3:28
$begingroup$
I have voted my question as duplicate. I have found the solution here math.stackexchange.com/questions/1850959/… I am not deleting it as it has received two upvotes and one favorites. Thanks
$endgroup$
– StammeringMathematician
Jan 31 at 3:28
1
1
$begingroup$
"Now if I put the initial value I get" What? Come again? What do you think the formula just after that even means?
$endgroup$
– Did
Jan 31 at 7:22
$begingroup$
"Now if I put the initial value I get" What? Come again? What do you think the formula just after that even means?
$endgroup$
– Did
Jan 31 at 7:22
$begingroup$
@Did Can you explain? I did not get it
$endgroup$
– StammeringMathematician
Jan 31 at 10:12
$begingroup$
@Did Can you explain? I did not get it
$endgroup$
– StammeringMathematician
Jan 31 at 10:12
1
1
$begingroup$
Again, what do you mean by "Now if I put the initial value I get $0=int e^{-1}dx+C$"? Can you explain? I do not get it.
$endgroup$
– Did
Jan 31 at 10:56
$begingroup$
Again, what do you mean by "Now if I put the initial value I get $0=int e^{-1}dx+C$"? Can you explain? I do not get it.
$endgroup$
– Did
Jan 31 at 10:56
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
In general terms, the solution of
$y' + Py = Q, ; y(-1) = 0, tag 1$
may be derived as follows:
$exp left (displaystyle int_{x_0}^x P(s); ds right ) y' + exp left (displaystyle int_{x_0}^x P(s); ds right ) Py = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 2$
$left ( exp left (displaystyle int_{x_0}^x P(s); ds right )y right )' = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 3$
$exp left (displaystyle int_{x_0}^x P(s); ds right )y(x) - y(x_0) =displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du; tag 4$
$y(x) = exp left (-displaystyle int_{x_0}^x P(s); ds right )left ( y(x_0) + displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du right ); tag 5$
in the present case,
$y' + y = vert x vert, ; y(-1) = 0; tag 6$
$P(x) = 1, ; Q(x) = vert x vert; tag 7$
$displaystyle int_{x_0}^x P(s) ; ds = x - x_0; tag 8$
$exp left ( displaystyle int_{x_0}^x P(s); ds right ) = e^{x - x_0}; tag 9$
with
$x_0 = -1, ; y(x_0) = y(-1) = 0 tag{10}$
we find
$exp left ( displaystyle int_{-1}^x P(s); ds right ) = e^{x + 1}, tag{11}$
$exp left (- displaystyle int_{-1}^x P(s); ds right ) = e^{-(x + 1)}, tag{12}$
and from (5),
$y(x) = e^{-(x + 1)} displaystyle int_{-1}^x e^{u + 1} vert u vert ; du = e^{-x} displaystyle int_{-1}^x e^u vert u vert ; du ; tag{13}$
with $x ge 0$,
$displaystyle int_{-1}^x e^u vert u vert ; du = int_{-1}^0 (-ue^u) ; du + int_0^x ue^u ; du = int_0^x ue^u ; du - int_{-1}^0 ue^u ; du; tag{14}$
the integrals of the form occurring in (14) are well-known; indeed the anti-derivative of $ue^u$ is found from
$(e^u(u - 1))' = e^u(u - 1) + e^u = ue^u; tag{15}$
it then follows that
$displaystyle int_a^b ue^e ; u = (e^u(u - 1)]_a^b =e^b(b - 1) - e^a(a - 1) = be^b - ae^a -(e^b - e^a); tag{16}$
we thus may evaluate (14):
$displaystyle int_0^x ue^u ; du - int_{-1}^0 ue^u ; du$
$= (xe^x - e^x + 1) - (e^{-1} -1 + e^{-1}) = xe^x - e^x + 2 - 2e^{-1}; tag{17}$
thus, from (13),
$y(x) = x - 1 + 2e^{-x} - 2e^{-1 - x}; tag{18}$
finally we find
$y(1) = 2e^{-1} - 2e^{-2}. tag{19}$
$endgroup$
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
In general terms, the solution of
$y' + Py = Q, ; y(-1) = 0, tag 1$
may be derived as follows:
$exp left (displaystyle int_{x_0}^x P(s); ds right ) y' + exp left (displaystyle int_{x_0}^x P(s); ds right ) Py = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 2$
$left ( exp left (displaystyle int_{x_0}^x P(s); ds right )y right )' = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 3$
$exp left (displaystyle int_{x_0}^x P(s); ds right )y(x) - y(x_0) =displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du; tag 4$
$y(x) = exp left (-displaystyle int_{x_0}^x P(s); ds right )left ( y(x_0) + displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du right ); tag 5$
in the present case,
$y' + y = vert x vert, ; y(-1) = 0; tag 6$
$P(x) = 1, ; Q(x) = vert x vert; tag 7$
$displaystyle int_{x_0}^x P(s) ; ds = x - x_0; tag 8$
$exp left ( displaystyle int_{x_0}^x P(s); ds right ) = e^{x - x_0}; tag 9$
with
$x_0 = -1, ; y(x_0) = y(-1) = 0 tag{10}$
we find
$exp left ( displaystyle int_{-1}^x P(s); ds right ) = e^{x + 1}, tag{11}$
$exp left (- displaystyle int_{-1}^x P(s); ds right ) = e^{-(x + 1)}, tag{12}$
and from (5),
$y(x) = e^{-(x + 1)} displaystyle int_{-1}^x e^{u + 1} vert u vert ; du = e^{-x} displaystyle int_{-1}^x e^u vert u vert ; du ; tag{13}$
with $x ge 0$,
$displaystyle int_{-1}^x e^u vert u vert ; du = int_{-1}^0 (-ue^u) ; du + int_0^x ue^u ; du = int_0^x ue^u ; du - int_{-1}^0 ue^u ; du; tag{14}$
the integrals of the form occurring in (14) are well-known; indeed the anti-derivative of $ue^u$ is found from
$(e^u(u - 1))' = e^u(u - 1) + e^u = ue^u; tag{15}$
it then follows that
$displaystyle int_a^b ue^e ; u = (e^u(u - 1)]_a^b =e^b(b - 1) - e^a(a - 1) = be^b - ae^a -(e^b - e^a); tag{16}$
we thus may evaluate (14):
$displaystyle int_0^x ue^u ; du - int_{-1}^0 ue^u ; du$
$= (xe^x - e^x + 1) - (e^{-1} -1 + e^{-1}) = xe^x - e^x + 2 - 2e^{-1}; tag{17}$
thus, from (13),
$y(x) = x - 1 + 2e^{-x} - 2e^{-1 - x}; tag{18}$
finally we find
$y(1) = 2e^{-1} - 2e^{-2}. tag{19}$
$endgroup$
add a comment |
$begingroup$
In general terms, the solution of
$y' + Py = Q, ; y(-1) = 0, tag 1$
may be derived as follows:
$exp left (displaystyle int_{x_0}^x P(s); ds right ) y' + exp left (displaystyle int_{x_0}^x P(s); ds right ) Py = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 2$
$left ( exp left (displaystyle int_{x_0}^x P(s); ds right )y right )' = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 3$
$exp left (displaystyle int_{x_0}^x P(s); ds right )y(x) - y(x_0) =displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du; tag 4$
$y(x) = exp left (-displaystyle int_{x_0}^x P(s); ds right )left ( y(x_0) + displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du right ); tag 5$
in the present case,
$y' + y = vert x vert, ; y(-1) = 0; tag 6$
$P(x) = 1, ; Q(x) = vert x vert; tag 7$
$displaystyle int_{x_0}^x P(s) ; ds = x - x_0; tag 8$
$exp left ( displaystyle int_{x_0}^x P(s); ds right ) = e^{x - x_0}; tag 9$
with
$x_0 = -1, ; y(x_0) = y(-1) = 0 tag{10}$
we find
$exp left ( displaystyle int_{-1}^x P(s); ds right ) = e^{x + 1}, tag{11}$
$exp left (- displaystyle int_{-1}^x P(s); ds right ) = e^{-(x + 1)}, tag{12}$
and from (5),
$y(x) = e^{-(x + 1)} displaystyle int_{-1}^x e^{u + 1} vert u vert ; du = e^{-x} displaystyle int_{-1}^x e^u vert u vert ; du ; tag{13}$
with $x ge 0$,
$displaystyle int_{-1}^x e^u vert u vert ; du = int_{-1}^0 (-ue^u) ; du + int_0^x ue^u ; du = int_0^x ue^u ; du - int_{-1}^0 ue^u ; du; tag{14}$
the integrals of the form occurring in (14) are well-known; indeed the anti-derivative of $ue^u$ is found from
$(e^u(u - 1))' = e^u(u - 1) + e^u = ue^u; tag{15}$
it then follows that
$displaystyle int_a^b ue^e ; u = (e^u(u - 1)]_a^b =e^b(b - 1) - e^a(a - 1) = be^b - ae^a -(e^b - e^a); tag{16}$
we thus may evaluate (14):
$displaystyle int_0^x ue^u ; du - int_{-1}^0 ue^u ; du$
$= (xe^x - e^x + 1) - (e^{-1} -1 + e^{-1}) = xe^x - e^x + 2 - 2e^{-1}; tag{17}$
thus, from (13),
$y(x) = x - 1 + 2e^{-x} - 2e^{-1 - x}; tag{18}$
finally we find
$y(1) = 2e^{-1} - 2e^{-2}. tag{19}$
$endgroup$
add a comment |
$begingroup$
In general terms, the solution of
$y' + Py = Q, ; y(-1) = 0, tag 1$
may be derived as follows:
$exp left (displaystyle int_{x_0}^x P(s); ds right ) y' + exp left (displaystyle int_{x_0}^x P(s); ds right ) Py = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 2$
$left ( exp left (displaystyle int_{x_0}^x P(s); ds right )y right )' = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 3$
$exp left (displaystyle int_{x_0}^x P(s); ds right )y(x) - y(x_0) =displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du; tag 4$
$y(x) = exp left (-displaystyle int_{x_0}^x P(s); ds right )left ( y(x_0) + displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du right ); tag 5$
in the present case,
$y' + y = vert x vert, ; y(-1) = 0; tag 6$
$P(x) = 1, ; Q(x) = vert x vert; tag 7$
$displaystyle int_{x_0}^x P(s) ; ds = x - x_0; tag 8$
$exp left ( displaystyle int_{x_0}^x P(s); ds right ) = e^{x - x_0}; tag 9$
with
$x_0 = -1, ; y(x_0) = y(-1) = 0 tag{10}$
we find
$exp left ( displaystyle int_{-1}^x P(s); ds right ) = e^{x + 1}, tag{11}$
$exp left (- displaystyle int_{-1}^x P(s); ds right ) = e^{-(x + 1)}, tag{12}$
and from (5),
$y(x) = e^{-(x + 1)} displaystyle int_{-1}^x e^{u + 1} vert u vert ; du = e^{-x} displaystyle int_{-1}^x e^u vert u vert ; du ; tag{13}$
with $x ge 0$,
$displaystyle int_{-1}^x e^u vert u vert ; du = int_{-1}^0 (-ue^u) ; du + int_0^x ue^u ; du = int_0^x ue^u ; du - int_{-1}^0 ue^u ; du; tag{14}$
the integrals of the form occurring in (14) are well-known; indeed the anti-derivative of $ue^u$ is found from
$(e^u(u - 1))' = e^u(u - 1) + e^u = ue^u; tag{15}$
it then follows that
$displaystyle int_a^b ue^e ; u = (e^u(u - 1)]_a^b =e^b(b - 1) - e^a(a - 1) = be^b - ae^a -(e^b - e^a); tag{16}$
we thus may evaluate (14):
$displaystyle int_0^x ue^u ; du - int_{-1}^0 ue^u ; du$
$= (xe^x - e^x + 1) - (e^{-1} -1 + e^{-1}) = xe^x - e^x + 2 - 2e^{-1}; tag{17}$
thus, from (13),
$y(x) = x - 1 + 2e^{-x} - 2e^{-1 - x}; tag{18}$
finally we find
$y(1) = 2e^{-1} - 2e^{-2}. tag{19}$
$endgroup$
In general terms, the solution of
$y' + Py = Q, ; y(-1) = 0, tag 1$
may be derived as follows:
$exp left (displaystyle int_{x_0}^x P(s); ds right ) y' + exp left (displaystyle int_{x_0}^x P(s); ds right ) Py = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 2$
$left ( exp left (displaystyle int_{x_0}^x P(s); ds right )y right )' = exp left (displaystyle int_{x_0}^x P(s); ds right )Q; tag 3$
$exp left (displaystyle int_{x_0}^x P(s); ds right )y(x) - y(x_0) =displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du; tag 4$
$y(x) = exp left (-displaystyle int_{x_0}^x P(s); ds right )left ( y(x_0) + displaystyle int_{x_0}^x exp left (displaystyle int_{x_0}^u P(s); ds right )Q(u) ; du right ); tag 5$
in the present case,
$y' + y = vert x vert, ; y(-1) = 0; tag 6$
$P(x) = 1, ; Q(x) = vert x vert; tag 7$
$displaystyle int_{x_0}^x P(s) ; ds = x - x_0; tag 8$
$exp left ( displaystyle int_{x_0}^x P(s); ds right ) = e^{x - x_0}; tag 9$
with
$x_0 = -1, ; y(x_0) = y(-1) = 0 tag{10}$
we find
$exp left ( displaystyle int_{-1}^x P(s); ds right ) = e^{x + 1}, tag{11}$
$exp left (- displaystyle int_{-1}^x P(s); ds right ) = e^{-(x + 1)}, tag{12}$
and from (5),
$y(x) = e^{-(x + 1)} displaystyle int_{-1}^x e^{u + 1} vert u vert ; du = e^{-x} displaystyle int_{-1}^x e^u vert u vert ; du ; tag{13}$
with $x ge 0$,
$displaystyle int_{-1}^x e^u vert u vert ; du = int_{-1}^0 (-ue^u) ; du + int_0^x ue^u ; du = int_0^x ue^u ; du - int_{-1}^0 ue^u ; du; tag{14}$
the integrals of the form occurring in (14) are well-known; indeed the anti-derivative of $ue^u$ is found from
$(e^u(u - 1))' = e^u(u - 1) + e^u = ue^u; tag{15}$
it then follows that
$displaystyle int_a^b ue^e ; u = (e^u(u - 1)]_a^b =e^b(b - 1) - e^a(a - 1) = be^b - ae^a -(e^b - e^a); tag{16}$
we thus may evaluate (14):
$displaystyle int_0^x ue^u ; du - int_{-1}^0 ue^u ; du$
$= (xe^x - e^x + 1) - (e^{-1} -1 + e^{-1}) = xe^x - e^x + 2 - 2e^{-1}; tag{17}$
thus, from (13),
$y(x) = x - 1 + 2e^{-x} - 2e^{-1 - x}; tag{18}$
finally we find
$y(1) = 2e^{-1} - 2e^{-2}. tag{19}$
edited Jan 31 at 5:51
answered Jan 31 at 5:03


Robert LewisRobert Lewis
48.6k23167
48.6k23167
add a comment |
add a comment |
$begingroup$
I have voted my question as duplicate. I have found the solution here math.stackexchange.com/questions/1850959/… I am not deleting it as it has received two upvotes and one favorites. Thanks
$endgroup$
– StammeringMathematician
Jan 31 at 3:28
1
$begingroup$
"Now if I put the initial value I get" What? Come again? What do you think the formula just after that even means?
$endgroup$
– Did
Jan 31 at 7:22
$begingroup$
@Did Can you explain? I did not get it
$endgroup$
– StammeringMathematician
Jan 31 at 10:12
1
$begingroup$
Again, what do you mean by "Now if I put the initial value I get $0=int e^{-1}dx+C$"? Can you explain? I do not get it.
$endgroup$
– Did
Jan 31 at 10:56