Stuck at indefinite integral
$begingroup$
I'm completely stuck on solving this indefinite integral:
$$intfrac{x-2}{-x^2+2x-5}dx$$
By completing the square in the denominator and separating the original into two integrals, I get:
$$-intfrac{x}{x^2-2x+5}dx -intfrac{2}{(x-1)^2 + 4}dx$$
The second one is trivial, but the first one has me stuck. Whatever substitution I apply or form I put it in, I just can't figure it out. They're meant to be solved without partial integration, by the way.
integration indefinite-integrals partial-fractions
$endgroup$
add a comment |
$begingroup$
I'm completely stuck on solving this indefinite integral:
$$intfrac{x-2}{-x^2+2x-5}dx$$
By completing the square in the denominator and separating the original into two integrals, I get:
$$-intfrac{x}{x^2-2x+5}dx -intfrac{2}{(x-1)^2 + 4}dx$$
The second one is trivial, but the first one has me stuck. Whatever substitution I apply or form I put it in, I just can't figure it out. They're meant to be solved without partial integration, by the way.
integration indefinite-integrals partial-fractions
$endgroup$
$begingroup$
Hint: look at the derivative of $ln (x^2-2x+5)$ to see how it helps.
$endgroup$
– J.G.
3 hours ago
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Let $u$ be the denominator and notice how $du$ nicely falls out.
$endgroup$
– John Douma
3 hours ago
$begingroup$
@JohnDouma That's what I tried, but if $u$ is the denominator, aren't I left with $intfrac{xdu}{(2x-2)u}$?
$endgroup$
– Arcturus
3 hours ago
$begingroup$
You could refer to an online integral calculator. integral-calculator.com It has a complete solution. It is a rather long solution, so I feel kind of lazy to type them out.
$endgroup$
– Larry
3 hours ago
add a comment |
$begingroup$
I'm completely stuck on solving this indefinite integral:
$$intfrac{x-2}{-x^2+2x-5}dx$$
By completing the square in the denominator and separating the original into two integrals, I get:
$$-intfrac{x}{x^2-2x+5}dx -intfrac{2}{(x-1)^2 + 4}dx$$
The second one is trivial, but the first one has me stuck. Whatever substitution I apply or form I put it in, I just can't figure it out. They're meant to be solved without partial integration, by the way.
integration indefinite-integrals partial-fractions
$endgroup$
I'm completely stuck on solving this indefinite integral:
$$intfrac{x-2}{-x^2+2x-5}dx$$
By completing the square in the denominator and separating the original into two integrals, I get:
$$-intfrac{x}{x^2-2x+5}dx -intfrac{2}{(x-1)^2 + 4}dx$$
The second one is trivial, but the first one has me stuck. Whatever substitution I apply or form I put it in, I just can't figure it out. They're meant to be solved without partial integration, by the way.
integration indefinite-integrals partial-fractions
integration indefinite-integrals partial-fractions
edited 4 mins ago
DavidG
1,988620
1,988620
asked 3 hours ago
ArcturusArcturus
695
695
$begingroup$
Hint: look at the derivative of $ln (x^2-2x+5)$ to see how it helps.
$endgroup$
– J.G.
3 hours ago
$begingroup$
Let $u$ be the denominator and notice how $du$ nicely falls out.
$endgroup$
– John Douma
3 hours ago
$begingroup$
@JohnDouma That's what I tried, but if $u$ is the denominator, aren't I left with $intfrac{xdu}{(2x-2)u}$?
$endgroup$
– Arcturus
3 hours ago
$begingroup$
You could refer to an online integral calculator. integral-calculator.com It has a complete solution. It is a rather long solution, so I feel kind of lazy to type them out.
$endgroup$
– Larry
3 hours ago
add a comment |
$begingroup$
Hint: look at the derivative of $ln (x^2-2x+5)$ to see how it helps.
$endgroup$
– J.G.
3 hours ago
$begingroup$
Let $u$ be the denominator and notice how $du$ nicely falls out.
$endgroup$
– John Douma
3 hours ago
$begingroup$
@JohnDouma That's what I tried, but if $u$ is the denominator, aren't I left with $intfrac{xdu}{(2x-2)u}$?
$endgroup$
– Arcturus
3 hours ago
$begingroup$
You could refer to an online integral calculator. integral-calculator.com It has a complete solution. It is a rather long solution, so I feel kind of lazy to type them out.
$endgroup$
– Larry
3 hours ago
$begingroup$
Hint: look at the derivative of $ln (x^2-2x+5)$ to see how it helps.
$endgroup$
– J.G.
3 hours ago
$begingroup$
Hint: look at the derivative of $ln (x^2-2x+5)$ to see how it helps.
$endgroup$
– J.G.
3 hours ago
$begingroup$
Let $u$ be the denominator and notice how $du$ nicely falls out.
$endgroup$
– John Douma
3 hours ago
$begingroup$
Let $u$ be the denominator and notice how $du$ nicely falls out.
$endgroup$
– John Douma
3 hours ago
$begingroup$
@JohnDouma That's what I tried, but if $u$ is the denominator, aren't I left with $intfrac{xdu}{(2x-2)u}$?
$endgroup$
– Arcturus
3 hours ago
$begingroup$
@JohnDouma That's what I tried, but if $u$ is the denominator, aren't I left with $intfrac{xdu}{(2x-2)u}$?
$endgroup$
– Arcturus
3 hours ago
$begingroup$
You could refer to an online integral calculator. integral-calculator.com It has a complete solution. It is a rather long solution, so I feel kind of lazy to type them out.
$endgroup$
– Larry
3 hours ago
$begingroup$
You could refer to an online integral calculator. integral-calculator.com It has a complete solution. It is a rather long solution, so I feel kind of lazy to type them out.
$endgroup$
– Larry
3 hours ago
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
The numerator of the integrand almost looks like the derivative of the denominator, which indicates that the solution might involve a logarithm. Let's follow our nose and see if we can arrive at such an expression (you have the right idea of splitting the integral!):
begin{align}intfrac{x-2}{-x^2+2x-5}dx &= - int frac{x-2}{x^2 - 2x + 5}dx
\&= -frac 12 int frac{2x-4}{x^2 - 2x + 5}dx. end{align} The numerator of the integrand is looking more like the derivative of the denominator, but not quite. Further manipulation yields
begin{align}
intfrac{x-2}{-x^2+2x-5}dx = -frac 12 left(int frac{2x-2}{x^2-2x+5}dx - int frac{2}{x^2-2x+5}dxright).
end{align}
The second integral you can solve, can you solve the first?
$endgroup$
1
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
add a comment |
$begingroup$
Another plan that may be useful: once we see that form with the completed square, we make a simple substitution - not the whole thing, but just the part inside the square.
begin{align*}I &= -int frac{x-2}{(x-1)^2+4},dx\
&phantom{|}^{u=x-1}_{du=dx}\
&= int -frac{u-1}{u^2+4},du = intfrac{-u}{u^2+4},du+intfrac{1}{u^2+4},duend{align*}
The substance of this is exactly the same as @E-mu's argument - the difference is how we arrive at the way to split the integrand. Instead of looking for the derivative of the denominator, we make an affine substitution so that the split becomes obvious.
$endgroup$
add a comment |
$begingroup$
$$I=intfrac{x-2}{-x^2+2x-5}dx$$
$$I=-intfrac{x-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2}{x^2-2x+5}dx+intfrac{dx}{x^2-2x+5}$$
$$I=-frac12I_1+I_2$$
$$I_1=intfrac{2x-2}{x^2-2x+5}dx$$
$u=x^2-2x+5Rightarrow du=(2x-2)dx$:
$$I_1=intfrac{du}u$$
$$I_1=ln|u|$$
$$I_1=ln|x^2-2x+5|$$
$$I_1=ln(x^2-2x+5)$$
$$I_2=text{something you know how to solve}$$
$$I=-frac12ln(x^2-2x+5)+I_2$$
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The numerator of the integrand almost looks like the derivative of the denominator, which indicates that the solution might involve a logarithm. Let's follow our nose and see if we can arrive at such an expression (you have the right idea of splitting the integral!):
begin{align}intfrac{x-2}{-x^2+2x-5}dx &= - int frac{x-2}{x^2 - 2x + 5}dx
\&= -frac 12 int frac{2x-4}{x^2 - 2x + 5}dx. end{align} The numerator of the integrand is looking more like the derivative of the denominator, but not quite. Further manipulation yields
begin{align}
intfrac{x-2}{-x^2+2x-5}dx = -frac 12 left(int frac{2x-2}{x^2-2x+5}dx - int frac{2}{x^2-2x+5}dxright).
end{align}
The second integral you can solve, can you solve the first?
$endgroup$
1
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
add a comment |
$begingroup$
The numerator of the integrand almost looks like the derivative of the denominator, which indicates that the solution might involve a logarithm. Let's follow our nose and see if we can arrive at such an expression (you have the right idea of splitting the integral!):
begin{align}intfrac{x-2}{-x^2+2x-5}dx &= - int frac{x-2}{x^2 - 2x + 5}dx
\&= -frac 12 int frac{2x-4}{x^2 - 2x + 5}dx. end{align} The numerator of the integrand is looking more like the derivative of the denominator, but not quite. Further manipulation yields
begin{align}
intfrac{x-2}{-x^2+2x-5}dx = -frac 12 left(int frac{2x-2}{x^2-2x+5}dx - int frac{2}{x^2-2x+5}dxright).
end{align}
The second integral you can solve, can you solve the first?
$endgroup$
1
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
add a comment |
$begingroup$
The numerator of the integrand almost looks like the derivative of the denominator, which indicates that the solution might involve a logarithm. Let's follow our nose and see if we can arrive at such an expression (you have the right idea of splitting the integral!):
begin{align}intfrac{x-2}{-x^2+2x-5}dx &= - int frac{x-2}{x^2 - 2x + 5}dx
\&= -frac 12 int frac{2x-4}{x^2 - 2x + 5}dx. end{align} The numerator of the integrand is looking more like the derivative of the denominator, but not quite. Further manipulation yields
begin{align}
intfrac{x-2}{-x^2+2x-5}dx = -frac 12 left(int frac{2x-2}{x^2-2x+5}dx - int frac{2}{x^2-2x+5}dxright).
end{align}
The second integral you can solve, can you solve the first?
$endgroup$
The numerator of the integrand almost looks like the derivative of the denominator, which indicates that the solution might involve a logarithm. Let's follow our nose and see if we can arrive at such an expression (you have the right idea of splitting the integral!):
begin{align}intfrac{x-2}{-x^2+2x-5}dx &= - int frac{x-2}{x^2 - 2x + 5}dx
\&= -frac 12 int frac{2x-4}{x^2 - 2x + 5}dx. end{align} The numerator of the integrand is looking more like the derivative of the denominator, but not quite. Further manipulation yields
begin{align}
intfrac{x-2}{-x^2+2x-5}dx = -frac 12 left(int frac{2x-2}{x^2-2x+5}dx - int frac{2}{x^2-2x+5}dxright).
end{align}
The second integral you can solve, can you solve the first?
edited 3 hours ago
answered 3 hours ago
E-muE-mu
620314
620314
1
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
add a comment |
1
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
1
1
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
$begingroup$
Thank you for a concise and very easy to understand answer. It's an ultimately simple trick of manipulating the expression to your favor, but the hard part is thinking of it. Well, hard at this late hour for me, at least :D Cheers mate. This will make any following similar problems that much easier to solve.
$endgroup$
– Arcturus
2 hours ago
add a comment |
$begingroup$
Another plan that may be useful: once we see that form with the completed square, we make a simple substitution - not the whole thing, but just the part inside the square.
begin{align*}I &= -int frac{x-2}{(x-1)^2+4},dx\
&phantom{|}^{u=x-1}_{du=dx}\
&= int -frac{u-1}{u^2+4},du = intfrac{-u}{u^2+4},du+intfrac{1}{u^2+4},duend{align*}
The substance of this is exactly the same as @E-mu's argument - the difference is how we arrive at the way to split the integrand. Instead of looking for the derivative of the denominator, we make an affine substitution so that the split becomes obvious.
$endgroup$
add a comment |
$begingroup$
Another plan that may be useful: once we see that form with the completed square, we make a simple substitution - not the whole thing, but just the part inside the square.
begin{align*}I &= -int frac{x-2}{(x-1)^2+4},dx\
&phantom{|}^{u=x-1}_{du=dx}\
&= int -frac{u-1}{u^2+4},du = intfrac{-u}{u^2+4},du+intfrac{1}{u^2+4},duend{align*}
The substance of this is exactly the same as @E-mu's argument - the difference is how we arrive at the way to split the integrand. Instead of looking for the derivative of the denominator, we make an affine substitution so that the split becomes obvious.
$endgroup$
add a comment |
$begingroup$
Another plan that may be useful: once we see that form with the completed square, we make a simple substitution - not the whole thing, but just the part inside the square.
begin{align*}I &= -int frac{x-2}{(x-1)^2+4},dx\
&phantom{|}^{u=x-1}_{du=dx}\
&= int -frac{u-1}{u^2+4},du = intfrac{-u}{u^2+4},du+intfrac{1}{u^2+4},duend{align*}
The substance of this is exactly the same as @E-mu's argument - the difference is how we arrive at the way to split the integrand. Instead of looking for the derivative of the denominator, we make an affine substitution so that the split becomes obvious.
$endgroup$
Another plan that may be useful: once we see that form with the completed square, we make a simple substitution - not the whole thing, but just the part inside the square.
begin{align*}I &= -int frac{x-2}{(x-1)^2+4},dx\
&phantom{|}^{u=x-1}_{du=dx}\
&= int -frac{u-1}{u^2+4},du = intfrac{-u}{u^2+4},du+intfrac{1}{u^2+4},duend{align*}
The substance of this is exactly the same as @E-mu's argument - the difference is how we arrive at the way to split the integrand. Instead of looking for the derivative of the denominator, we make an affine substitution so that the split becomes obvious.
answered 2 hours ago
jmerryjmerry
3,409413
3,409413
add a comment |
add a comment |
$begingroup$
$$I=intfrac{x-2}{-x^2+2x-5}dx$$
$$I=-intfrac{x-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2}{x^2-2x+5}dx+intfrac{dx}{x^2-2x+5}$$
$$I=-frac12I_1+I_2$$
$$I_1=intfrac{2x-2}{x^2-2x+5}dx$$
$u=x^2-2x+5Rightarrow du=(2x-2)dx$:
$$I_1=intfrac{du}u$$
$$I_1=ln|u|$$
$$I_1=ln|x^2-2x+5|$$
$$I_1=ln(x^2-2x+5)$$
$$I_2=text{something you know how to solve}$$
$$I=-frac12ln(x^2-2x+5)+I_2$$
$endgroup$
add a comment |
$begingroup$
$$I=intfrac{x-2}{-x^2+2x-5}dx$$
$$I=-intfrac{x-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2}{x^2-2x+5}dx+intfrac{dx}{x^2-2x+5}$$
$$I=-frac12I_1+I_2$$
$$I_1=intfrac{2x-2}{x^2-2x+5}dx$$
$u=x^2-2x+5Rightarrow du=(2x-2)dx$:
$$I_1=intfrac{du}u$$
$$I_1=ln|u|$$
$$I_1=ln|x^2-2x+5|$$
$$I_1=ln(x^2-2x+5)$$
$$I_2=text{something you know how to solve}$$
$$I=-frac12ln(x^2-2x+5)+I_2$$
$endgroup$
add a comment |
$begingroup$
$$I=intfrac{x-2}{-x^2+2x-5}dx$$
$$I=-intfrac{x-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2}{x^2-2x+5}dx+intfrac{dx}{x^2-2x+5}$$
$$I=-frac12I_1+I_2$$
$$I_1=intfrac{2x-2}{x^2-2x+5}dx$$
$u=x^2-2x+5Rightarrow du=(2x-2)dx$:
$$I_1=intfrac{du}u$$
$$I_1=ln|u|$$
$$I_1=ln|x^2-2x+5|$$
$$I_1=ln(x^2-2x+5)$$
$$I_2=text{something you know how to solve}$$
$$I=-frac12ln(x^2-2x+5)+I_2$$
$endgroup$
$$I=intfrac{x-2}{-x^2+2x-5}dx$$
$$I=-intfrac{x-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2-2}{x^2-2x+5}dx$$
$$I=-frac12intfrac{2x-2}{x^2-2x+5}dx+intfrac{dx}{x^2-2x+5}$$
$$I=-frac12I_1+I_2$$
$$I_1=intfrac{2x-2}{x^2-2x+5}dx$$
$u=x^2-2x+5Rightarrow du=(2x-2)dx$:
$$I_1=intfrac{du}u$$
$$I_1=ln|u|$$
$$I_1=ln|x^2-2x+5|$$
$$I_1=ln(x^2-2x+5)$$
$$I_2=text{something you know how to solve}$$
$$I=-frac12ln(x^2-2x+5)+I_2$$
answered 3 hours ago
clathratusclathratus
3,541332
3,541332
add a comment |
add a comment |
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$begingroup$
Hint: look at the derivative of $ln (x^2-2x+5)$ to see how it helps.
$endgroup$
– J.G.
3 hours ago
$begingroup$
Let $u$ be the denominator and notice how $du$ nicely falls out.
$endgroup$
– John Douma
3 hours ago
$begingroup$
@JohnDouma That's what I tried, but if $u$ is the denominator, aren't I left with $intfrac{xdu}{(2x-2)u}$?
$endgroup$
– Arcturus
3 hours ago
$begingroup$
You could refer to an online integral calculator. integral-calculator.com It has a complete solution. It is a rather long solution, so I feel kind of lazy to type them out.
$endgroup$
– Larry
3 hours ago