Integral check. Is partial fractions the only way?
$begingroup$
I'm getting a different answer from wolfram and I have no idea where. I have to integrate:
$$int_0^1 frac{xdx}{(2x+1)^3}$$
Is partial fractions the only way?
So evaluating the fraction first:
$$frac{x}{(2x+1)^3} = frac{A}{2x+1} + frac{B}{(2x+1)^2} + frac{C}{(2x+1)^3}$$
$$x = A(2x+1)^2 + B(2x+1) + C$$
$$x = A(4x^2 + 4x + 1) + 2Bx + B + C$$
$$x = 4Ax^2 + 4Ax + A + 2Bx + B + C$$
$$x = x^2(4A) + x(4A+2B) + A + B + C$$
$4A = 0$ and $4A+2B = 1$ and $A + B + C = 0$ so $A = 0$ and $B=frac{1}{2}$ and $C = frac{-1}{2}$
Is the partial fraction part right?
So then I get:
$$int_0^1 frac{xdx}{(2x+1)^3} = int_0^1 frac{dx}{(2(2x+1)^2)} - int_0^1 frac{dx}{2(2x+1)^3}$$
for both, using $u = 2x+1$ and $frac{du}{dx} = 2$ and $du = 2dx$ and $frac{du}{2} = dx$,
$$frac{1}{4} int frac{du}{u^2} - frac{1}{4} int frac{du}{u^3}$$
$$ = [frac{1}{4} - u^{-1} - frac{1}{4} cdot frac{1}{-2} u^{-2} ]_1^3$$
I'm going the route of changing the limits to the new u and since $u = 2x+1$, when $x = 0, u = 1$ and when $x = 1, u = 3$. Is this the right path?
finally I get
$$[frac{-1}{4}u^{-1} + frac{1}{8}u^{-2} ]_1^3$$
I plug in numbers but I get a different answer than wolfram...
integration
$endgroup$
add a comment |
$begingroup$
I'm getting a different answer from wolfram and I have no idea where. I have to integrate:
$$int_0^1 frac{xdx}{(2x+1)^3}$$
Is partial fractions the only way?
So evaluating the fraction first:
$$frac{x}{(2x+1)^3} = frac{A}{2x+1} + frac{B}{(2x+1)^2} + frac{C}{(2x+1)^3}$$
$$x = A(2x+1)^2 + B(2x+1) + C$$
$$x = A(4x^2 + 4x + 1) + 2Bx + B + C$$
$$x = 4Ax^2 + 4Ax + A + 2Bx + B + C$$
$$x = x^2(4A) + x(4A+2B) + A + B + C$$
$4A = 0$ and $4A+2B = 1$ and $A + B + C = 0$ so $A = 0$ and $B=frac{1}{2}$ and $C = frac{-1}{2}$
Is the partial fraction part right?
So then I get:
$$int_0^1 frac{xdx}{(2x+1)^3} = int_0^1 frac{dx}{(2(2x+1)^2)} - int_0^1 frac{dx}{2(2x+1)^3}$$
for both, using $u = 2x+1$ and $frac{du}{dx} = 2$ and $du = 2dx$ and $frac{du}{2} = dx$,
$$frac{1}{4} int frac{du}{u^2} - frac{1}{4} int frac{du}{u^3}$$
$$ = [frac{1}{4} - u^{-1} - frac{1}{4} cdot frac{1}{-2} u^{-2} ]_1^3$$
I'm going the route of changing the limits to the new u and since $u = 2x+1$, when $x = 0, u = 1$ and when $x = 1, u = 3$. Is this the right path?
finally I get
$$[frac{-1}{4}u^{-1} + frac{1}{8}u^{-2} ]_1^3$$
I plug in numbers but I get a different answer than wolfram...
integration
$endgroup$
$begingroup$
I can't see any mistake in your work. What's the answer given by wolfram?
$endgroup$
– Thomas Shelby
2 hours ago
$begingroup$
They have 1/18 as the answer @ThomasShelby
$endgroup$
– Jwan622
42 mins ago
add a comment |
$begingroup$
I'm getting a different answer from wolfram and I have no idea where. I have to integrate:
$$int_0^1 frac{xdx}{(2x+1)^3}$$
Is partial fractions the only way?
So evaluating the fraction first:
$$frac{x}{(2x+1)^3} = frac{A}{2x+1} + frac{B}{(2x+1)^2} + frac{C}{(2x+1)^3}$$
$$x = A(2x+1)^2 + B(2x+1) + C$$
$$x = A(4x^2 + 4x + 1) + 2Bx + B + C$$
$$x = 4Ax^2 + 4Ax + A + 2Bx + B + C$$
$$x = x^2(4A) + x(4A+2B) + A + B + C$$
$4A = 0$ and $4A+2B = 1$ and $A + B + C = 0$ so $A = 0$ and $B=frac{1}{2}$ and $C = frac{-1}{2}$
Is the partial fraction part right?
So then I get:
$$int_0^1 frac{xdx}{(2x+1)^3} = int_0^1 frac{dx}{(2(2x+1)^2)} - int_0^1 frac{dx}{2(2x+1)^3}$$
for both, using $u = 2x+1$ and $frac{du}{dx} = 2$ and $du = 2dx$ and $frac{du}{2} = dx$,
$$frac{1}{4} int frac{du}{u^2} - frac{1}{4} int frac{du}{u^3}$$
$$ = [frac{1}{4} - u^{-1} - frac{1}{4} cdot frac{1}{-2} u^{-2} ]_1^3$$
I'm going the route of changing the limits to the new u and since $u = 2x+1$, when $x = 0, u = 1$ and when $x = 1, u = 3$. Is this the right path?
finally I get
$$[frac{-1}{4}u^{-1} + frac{1}{8}u^{-2} ]_1^3$$
I plug in numbers but I get a different answer than wolfram...
integration
$endgroup$
I'm getting a different answer from wolfram and I have no idea where. I have to integrate:
$$int_0^1 frac{xdx}{(2x+1)^3}$$
Is partial fractions the only way?
So evaluating the fraction first:
$$frac{x}{(2x+1)^3} = frac{A}{2x+1} + frac{B}{(2x+1)^2} + frac{C}{(2x+1)^3}$$
$$x = A(2x+1)^2 + B(2x+1) + C$$
$$x = A(4x^2 + 4x + 1) + 2Bx + B + C$$
$$x = 4Ax^2 + 4Ax + A + 2Bx + B + C$$
$$x = x^2(4A) + x(4A+2B) + A + B + C$$
$4A = 0$ and $4A+2B = 1$ and $A + B + C = 0$ so $A = 0$ and $B=frac{1}{2}$ and $C = frac{-1}{2}$
Is the partial fraction part right?
So then I get:
$$int_0^1 frac{xdx}{(2x+1)^3} = int_0^1 frac{dx}{(2(2x+1)^2)} - int_0^1 frac{dx}{2(2x+1)^3}$$
for both, using $u = 2x+1$ and $frac{du}{dx} = 2$ and $du = 2dx$ and $frac{du}{2} = dx$,
$$frac{1}{4} int frac{du}{u^2} - frac{1}{4} int frac{du}{u^3}$$
$$ = [frac{1}{4} - u^{-1} - frac{1}{4} cdot frac{1}{-2} u^{-2} ]_1^3$$
I'm going the route of changing the limits to the new u and since $u = 2x+1$, when $x = 0, u = 1$ and when $x = 1, u = 3$. Is this the right path?
finally I get
$$[frac{-1}{4}u^{-1} + frac{1}{8}u^{-2} ]_1^3$$
I plug in numbers but I get a different answer than wolfram...
integration
integration
edited 2 hours ago
Joshua Mundinger
2,5191028
2,5191028
asked 2 hours ago
Jwan622Jwan622
2,13611530
2,13611530
$begingroup$
I can't see any mistake in your work. What's the answer given by wolfram?
$endgroup$
– Thomas Shelby
2 hours ago
$begingroup$
They have 1/18 as the answer @ThomasShelby
$endgroup$
– Jwan622
42 mins ago
add a comment |
$begingroup$
I can't see any mistake in your work. What's the answer given by wolfram?
$endgroup$
– Thomas Shelby
2 hours ago
$begingroup$
They have 1/18 as the answer @ThomasShelby
$endgroup$
– Jwan622
42 mins ago
$begingroup$
I can't see any mistake in your work. What's the answer given by wolfram?
$endgroup$
– Thomas Shelby
2 hours ago
$begingroup$
I can't see any mistake in your work. What's the answer given by wolfram?
$endgroup$
– Thomas Shelby
2 hours ago
$begingroup$
They have 1/18 as the answer @ThomasShelby
$endgroup$
– Jwan622
42 mins ago
$begingroup$
They have 1/18 as the answer @ThomasShelby
$endgroup$
– Jwan622
42 mins ago
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Hint: Substitute $x=tfrac12 u -tfrac12$
$endgroup$
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
add a comment |
$begingroup$
A much much easier way to solve it is by using integration by parts.
Hint: Take $u=x$, $dv = frac{dx}{(2x+1)^3}$.
$endgroup$
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
add a comment |
$begingroup$
1) Observe: $$x= frac 12 times left ( (2x + 1) -1right).$$
2) Use this to obtain $$frac{x}{(2x+1)^3} = frac{1}{2} times frac{1}{(2x+1)^2} - frac 12 times frac{1}{(2x+1)^3}.$$
3) Integrate to get
$$ int frac{x}{(2x+1)^3} dx = -frac{1}{4} (2x+1)^{-1} + frac{1}{8} (2x+1)^{-2}.$$
$endgroup$
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%2f3120889%2fintegral-check-is-partial-fractions-the-only-way%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint: Substitute $x=tfrac12 u -tfrac12$
$endgroup$
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
add a comment |
$begingroup$
Hint: Substitute $x=tfrac12 u -tfrac12$
$endgroup$
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
add a comment |
$begingroup$
Hint: Substitute $x=tfrac12 u -tfrac12$
$endgroup$
Hint: Substitute $x=tfrac12 u -tfrac12$
answered 2 hours ago
MPWMPW
30.3k12157
30.3k12157
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
add a comment |
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
You beat me to it.
$endgroup$
– randomgirl
2 hours ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
$begingroup$
If I go your way, I wind up with $frac{1}{2} int frac{1}{u^2} - frac{1}{2} int frac{1}{u^3}$ which is different than what I have at some point. I can't see my error.
$endgroup$
– Jwan622
19 mins ago
add a comment |
$begingroup$
A much much easier way to solve it is by using integration by parts.
Hint: Take $u=x$, $dv = frac{dx}{(2x+1)^3}$.
$endgroup$
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
add a comment |
$begingroup$
A much much easier way to solve it is by using integration by parts.
Hint: Take $u=x$, $dv = frac{dx}{(2x+1)^3}$.
$endgroup$
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
add a comment |
$begingroup$
A much much easier way to solve it is by using integration by parts.
Hint: Take $u=x$, $dv = frac{dx}{(2x+1)^3}$.
$endgroup$
A much much easier way to solve it is by using integration by parts.
Hint: Take $u=x$, $dv = frac{dx}{(2x+1)^3}$.
answered 2 hours ago
Haris GusicHaris Gusic
960116
960116
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
add a comment |
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
$u$ but then $dv$?
$endgroup$
– manooooh
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
@manooooh What do you mean? I don't understand.
$endgroup$
– Haris Gusic
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
$begingroup$
I am sorry, I understood that you used sub. My apologies.
$endgroup$
– manooooh
2 hours ago
add a comment |
$begingroup$
1) Observe: $$x= frac 12 times left ( (2x + 1) -1right).$$
2) Use this to obtain $$frac{x}{(2x+1)^3} = frac{1}{2} times frac{1}{(2x+1)^2} - frac 12 times frac{1}{(2x+1)^3}.$$
3) Integrate to get
$$ int frac{x}{(2x+1)^3} dx = -frac{1}{4} (2x+1)^{-1} + frac{1}{8} (2x+1)^{-2}.$$
$endgroup$
add a comment |
$begingroup$
1) Observe: $$x= frac 12 times left ( (2x + 1) -1right).$$
2) Use this to obtain $$frac{x}{(2x+1)^3} = frac{1}{2} times frac{1}{(2x+1)^2} - frac 12 times frac{1}{(2x+1)^3}.$$
3) Integrate to get
$$ int frac{x}{(2x+1)^3} dx = -frac{1}{4} (2x+1)^{-1} + frac{1}{8} (2x+1)^{-2}.$$
$endgroup$
add a comment |
$begingroup$
1) Observe: $$x= frac 12 times left ( (2x + 1) -1right).$$
2) Use this to obtain $$frac{x}{(2x+1)^3} = frac{1}{2} times frac{1}{(2x+1)^2} - frac 12 times frac{1}{(2x+1)^3}.$$
3) Integrate to get
$$ int frac{x}{(2x+1)^3} dx = -frac{1}{4} (2x+1)^{-1} + frac{1}{8} (2x+1)^{-2}.$$
$endgroup$
1) Observe: $$x= frac 12 times left ( (2x + 1) -1right).$$
2) Use this to obtain $$frac{x}{(2x+1)^3} = frac{1}{2} times frac{1}{(2x+1)^2} - frac 12 times frac{1}{(2x+1)^3}.$$
3) Integrate to get
$$ int frac{x}{(2x+1)^3} dx = -frac{1}{4} (2x+1)^{-1} + frac{1}{8} (2x+1)^{-2}.$$
answered 2 hours ago
FnacoolFnacool
5,021511
5,021511
add a comment |
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%2f3120889%2fintegral-check-is-partial-fractions-the-only-way%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
$begingroup$
I can't see any mistake in your work. What's the answer given by wolfram?
$endgroup$
– Thomas Shelby
2 hours ago
$begingroup$
They have 1/18 as the answer @ThomasShelby
$endgroup$
– Jwan622
42 mins ago