Integral utter confusion with substition and dx/du
$begingroup$
I need to find the indefinite integral I = $$int e^x (1+e^x)^{frac{1}{2}}$$
by using a proper substition method. I tried it on https://www.integral-calculator.com and it gave the following explanation
Now this is where it gets confusing, when substituting $u = (1+e^x)$ I would suspect we got $$int e^x sqrt{(1+e^x)} = int e^xsqrt{u} $$
But that's not what the screenshot says.
It says that the original problem equals $int sqrt{u} du$ and also that $dx = e^{-x}$. Now where does this $e^{-x}$ come from? The antiderivite of $e^x$ is still $e^x$. And the antiderivative of $e^x+1$ is $e^x+x$ so where does that negative number come from and why is it used?
So those are my questions.
1. Why is $$int e^x sqrt{(1+e^x)} = int sqrt{u} du $$
2. Where does the $dx = e^{-x}$ come from
3. WHy is it even necessary?
integration indefinite-integrals substitution
$endgroup$
add a comment |
$begingroup$
I need to find the indefinite integral I = $$int e^x (1+e^x)^{frac{1}{2}}$$
by using a proper substition method. I tried it on https://www.integral-calculator.com and it gave the following explanation
Now this is where it gets confusing, when substituting $u = (1+e^x)$ I would suspect we got $$int e^x sqrt{(1+e^x)} = int e^xsqrt{u} $$
But that's not what the screenshot says.
It says that the original problem equals $int sqrt{u} du$ and also that $dx = e^{-x}$. Now where does this $e^{-x}$ come from? The antiderivite of $e^x$ is still $e^x$. And the antiderivative of $e^x+1$ is $e^x+x$ so where does that negative number come from and why is it used?
So those are my questions.
1. Why is $$int e^x sqrt{(1+e^x)} = int sqrt{u} du $$
2. Where does the $dx = e^{-x}$ come from
3. WHy is it even necessary?
integration indefinite-integrals substitution
$endgroup$
$begingroup$
You should not forget about writing $dx $ every time you deal with an integral.
$endgroup$
– user
Jan 10 at 20:46
add a comment |
$begingroup$
I need to find the indefinite integral I = $$int e^x (1+e^x)^{frac{1}{2}}$$
by using a proper substition method. I tried it on https://www.integral-calculator.com and it gave the following explanation
Now this is where it gets confusing, when substituting $u = (1+e^x)$ I would suspect we got $$int e^x sqrt{(1+e^x)} = int e^xsqrt{u} $$
But that's not what the screenshot says.
It says that the original problem equals $int sqrt{u} du$ and also that $dx = e^{-x}$. Now where does this $e^{-x}$ come from? The antiderivite of $e^x$ is still $e^x$. And the antiderivative of $e^x+1$ is $e^x+x$ so where does that negative number come from and why is it used?
So those are my questions.
1. Why is $$int e^x sqrt{(1+e^x)} = int sqrt{u} du $$
2. Where does the $dx = e^{-x}$ come from
3. WHy is it even necessary?
integration indefinite-integrals substitution
$endgroup$
I need to find the indefinite integral I = $$int e^x (1+e^x)^{frac{1}{2}}$$
by using a proper substition method. I tried it on https://www.integral-calculator.com and it gave the following explanation
Now this is where it gets confusing, when substituting $u = (1+e^x)$ I would suspect we got $$int e^x sqrt{(1+e^x)} = int e^xsqrt{u} $$
But that's not what the screenshot says.
It says that the original problem equals $int sqrt{u} du$ and also that $dx = e^{-x}$. Now where does this $e^{-x}$ come from? The antiderivite of $e^x$ is still $e^x$. And the antiderivative of $e^x+1$ is $e^x+x$ so where does that negative number come from and why is it used?
So those are my questions.
1. Why is $$int e^x sqrt{(1+e^x)} = int sqrt{u} du $$
2. Where does the $dx = e^{-x}$ come from
3. WHy is it even necessary?
integration indefinite-integrals substitution
integration indefinite-integrals substitution
asked Jan 10 at 20:14
Wouter VandenputteWouter Vandenputte
1295
1295
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You should not forget about writing $dx $ every time you deal with an integral.
$endgroup$
– user
Jan 10 at 20:46
add a comment |
$begingroup$
You should not forget about writing $dx $ every time you deal with an integral.
$endgroup$
– user
Jan 10 at 20:46
$begingroup$
You should not forget about writing $dx $ every time you deal with an integral.
$endgroup$
– user
Jan 10 at 20:46
$begingroup$
You should not forget about writing $dx $ every time you deal with an integral.
$endgroup$
– user
Jan 10 at 20:46
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
$(1)$ Because when you make the substitution $color{purple}{u = e^x+1}$, taking the derivative of $u$ yields $frac{du}{dx} = e^x$. Hence, $color{blue}{e^xdx = frac{du}{dx}dx = du}$. The integral can therefore be rewritten:
$$int color{blue}{e^x}sqrt{color{purple}{e^x+1}} color{blue}{dx} implies intsqrt{color{purple}{u}}color{blue}{du}$$
$(2)$ In the previous part, note that we get $frac{du}{dx} = e^x$. This can be rewritten as $frac{du}{e^x} = dx$, and $e^x$ in the denominator can be written as $e^{-x}$ in the numerator, so $dx = e^{-x}du$.
$(3)$ It’s certainly not necessary. It makes the integral easier to evaluate.
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add a comment |
$begingroup$
If $e^{x}-1 =u rightarrow e^xdx=du$ (I suggest writing it this way) so the integral after the substitution looks like this:
$int (e^x sqrt{e^x-1})dx=int(sqrt{u})du=cfrac{2u^{3/2}}{3} + C=cfrac{2(e^x-1)^{3/2}}{3}+C$
$endgroup$
add a comment |
$begingroup$
The way I learnt it more then sixty years ago.
$$I=int e^x sqrt{e^x+1},dx$$
$$e^x+1=uimplies x=log(u-1) implies dx=frac{du}{u-1}$$ making
$$I=int (u-1), sqrt u ,frac{du}{u-1}=int sqrt u ,du=cdots$$
$endgroup$
add a comment |
$begingroup$
Another approach:
begin{equation}
I = int e^x sqrt{e^x + 1}:dx
end{equation}
Here let $u^2 = e^x + 1$. Employing Implicit Differentiation we see
begin{equation}
frac{d}{dx}left[u^2right] = frac{d}{dx}left[e^x + 1 right] rightarrow 2u frac{du}{dx} = e^x rightarrow dx = frac{2u}{e^x}:du = frac{2u}{u^2 - 1}:du
end{equation}
Thus:
begin{align}
I &= int e^x sqrt{e^x + 1}:dx = int left(u^2 - 1right)sqrt{u^2}frac{2u}{u^2 - 1}:du \
&= int 2u^2 :du = frac{u^3}{3} + C = frac{1}{3}left(e^x + 1 right)^frac{3}{2} + C
end{align}
Where $C$ is the constant of integration.
$endgroup$
add a comment |
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
$(1)$ Because when you make the substitution $color{purple}{u = e^x+1}$, taking the derivative of $u$ yields $frac{du}{dx} = e^x$. Hence, $color{blue}{e^xdx = frac{du}{dx}dx = du}$. The integral can therefore be rewritten:
$$int color{blue}{e^x}sqrt{color{purple}{e^x+1}} color{blue}{dx} implies intsqrt{color{purple}{u}}color{blue}{du}$$
$(2)$ In the previous part, note that we get $frac{du}{dx} = e^x$. This can be rewritten as $frac{du}{e^x} = dx$, and $e^x$ in the denominator can be written as $e^{-x}$ in the numerator, so $dx = e^{-x}du$.
$(3)$ It’s certainly not necessary. It makes the integral easier to evaluate.
$endgroup$
add a comment |
$begingroup$
$(1)$ Because when you make the substitution $color{purple}{u = e^x+1}$, taking the derivative of $u$ yields $frac{du}{dx} = e^x$. Hence, $color{blue}{e^xdx = frac{du}{dx}dx = du}$. The integral can therefore be rewritten:
$$int color{blue}{e^x}sqrt{color{purple}{e^x+1}} color{blue}{dx} implies intsqrt{color{purple}{u}}color{blue}{du}$$
$(2)$ In the previous part, note that we get $frac{du}{dx} = e^x$. This can be rewritten as $frac{du}{e^x} = dx$, and $e^x$ in the denominator can be written as $e^{-x}$ in the numerator, so $dx = e^{-x}du$.
$(3)$ It’s certainly not necessary. It makes the integral easier to evaluate.
$endgroup$
add a comment |
$begingroup$
$(1)$ Because when you make the substitution $color{purple}{u = e^x+1}$, taking the derivative of $u$ yields $frac{du}{dx} = e^x$. Hence, $color{blue}{e^xdx = frac{du}{dx}dx = du}$. The integral can therefore be rewritten:
$$int color{blue}{e^x}sqrt{color{purple}{e^x+1}} color{blue}{dx} implies intsqrt{color{purple}{u}}color{blue}{du}$$
$(2)$ In the previous part, note that we get $frac{du}{dx} = e^x$. This can be rewritten as $frac{du}{e^x} = dx$, and $e^x$ in the denominator can be written as $e^{-x}$ in the numerator, so $dx = e^{-x}du$.
$(3)$ It’s certainly not necessary. It makes the integral easier to evaluate.
$endgroup$
$(1)$ Because when you make the substitution $color{purple}{u = e^x+1}$, taking the derivative of $u$ yields $frac{du}{dx} = e^x$. Hence, $color{blue}{e^xdx = frac{du}{dx}dx = du}$. The integral can therefore be rewritten:
$$int color{blue}{e^x}sqrt{color{purple}{e^x+1}} color{blue}{dx} implies intsqrt{color{purple}{u}}color{blue}{du}$$
$(2)$ In the previous part, note that we get $frac{du}{dx} = e^x$. This can be rewritten as $frac{du}{e^x} = dx$, and $e^x$ in the denominator can be written as $e^{-x}$ in the numerator, so $dx = e^{-x}du$.
$(3)$ It’s certainly not necessary. It makes the integral easier to evaluate.
answered Jan 10 at 20:24
KM101KM101
6,0861525
6,0861525
add a comment |
add a comment |
$begingroup$
If $e^{x}-1 =u rightarrow e^xdx=du$ (I suggest writing it this way) so the integral after the substitution looks like this:
$int (e^x sqrt{e^x-1})dx=int(sqrt{u})du=cfrac{2u^{3/2}}{3} + C=cfrac{2(e^x-1)^{3/2}}{3}+C$
$endgroup$
add a comment |
$begingroup$
If $e^{x}-1 =u rightarrow e^xdx=du$ (I suggest writing it this way) so the integral after the substitution looks like this:
$int (e^x sqrt{e^x-1})dx=int(sqrt{u})du=cfrac{2u^{3/2}}{3} + C=cfrac{2(e^x-1)^{3/2}}{3}+C$
$endgroup$
add a comment |
$begingroup$
If $e^{x}-1 =u rightarrow e^xdx=du$ (I suggest writing it this way) so the integral after the substitution looks like this:
$int (e^x sqrt{e^x-1})dx=int(sqrt{u})du=cfrac{2u^{3/2}}{3} + C=cfrac{2(e^x-1)^{3/2}}{3}+C$
$endgroup$
If $e^{x}-1 =u rightarrow e^xdx=du$ (I suggest writing it this way) so the integral after the substitution looks like this:
$int (e^x sqrt{e^x-1})dx=int(sqrt{u})du=cfrac{2u^{3/2}}{3} + C=cfrac{2(e^x-1)^{3/2}}{3}+C$
edited Jan 10 at 20:29
answered Jan 10 at 20:21
Rafał SzypulskiRafał Szypulski
35015
35015
add a comment |
add a comment |
$begingroup$
The way I learnt it more then sixty years ago.
$$I=int e^x sqrt{e^x+1},dx$$
$$e^x+1=uimplies x=log(u-1) implies dx=frac{du}{u-1}$$ making
$$I=int (u-1), sqrt u ,frac{du}{u-1}=int sqrt u ,du=cdots$$
$endgroup$
add a comment |
$begingroup$
The way I learnt it more then sixty years ago.
$$I=int e^x sqrt{e^x+1},dx$$
$$e^x+1=uimplies x=log(u-1) implies dx=frac{du}{u-1}$$ making
$$I=int (u-1), sqrt u ,frac{du}{u-1}=int sqrt u ,du=cdots$$
$endgroup$
add a comment |
$begingroup$
The way I learnt it more then sixty years ago.
$$I=int e^x sqrt{e^x+1},dx$$
$$e^x+1=uimplies x=log(u-1) implies dx=frac{du}{u-1}$$ making
$$I=int (u-1), sqrt u ,frac{du}{u-1}=int sqrt u ,du=cdots$$
$endgroup$
The way I learnt it more then sixty years ago.
$$I=int e^x sqrt{e^x+1},dx$$
$$e^x+1=uimplies x=log(u-1) implies dx=frac{du}{u-1}$$ making
$$I=int (u-1), sqrt u ,frac{du}{u-1}=int sqrt u ,du=cdots$$
answered Jan 11 at 7:03
Claude LeiboviciClaude Leibovici
125k1158135
125k1158135
add a comment |
add a comment |
$begingroup$
Another approach:
begin{equation}
I = int e^x sqrt{e^x + 1}:dx
end{equation}
Here let $u^2 = e^x + 1$. Employing Implicit Differentiation we see
begin{equation}
frac{d}{dx}left[u^2right] = frac{d}{dx}left[e^x + 1 right] rightarrow 2u frac{du}{dx} = e^x rightarrow dx = frac{2u}{e^x}:du = frac{2u}{u^2 - 1}:du
end{equation}
Thus:
begin{align}
I &= int e^x sqrt{e^x + 1}:dx = int left(u^2 - 1right)sqrt{u^2}frac{2u}{u^2 - 1}:du \
&= int 2u^2 :du = frac{u^3}{3} + C = frac{1}{3}left(e^x + 1 right)^frac{3}{2} + C
end{align}
Where $C$ is the constant of integration.
$endgroup$
add a comment |
$begingroup$
Another approach:
begin{equation}
I = int e^x sqrt{e^x + 1}:dx
end{equation}
Here let $u^2 = e^x + 1$. Employing Implicit Differentiation we see
begin{equation}
frac{d}{dx}left[u^2right] = frac{d}{dx}left[e^x + 1 right] rightarrow 2u frac{du}{dx} = e^x rightarrow dx = frac{2u}{e^x}:du = frac{2u}{u^2 - 1}:du
end{equation}
Thus:
begin{align}
I &= int e^x sqrt{e^x + 1}:dx = int left(u^2 - 1right)sqrt{u^2}frac{2u}{u^2 - 1}:du \
&= int 2u^2 :du = frac{u^3}{3} + C = frac{1}{3}left(e^x + 1 right)^frac{3}{2} + C
end{align}
Where $C$ is the constant of integration.
$endgroup$
add a comment |
$begingroup$
Another approach:
begin{equation}
I = int e^x sqrt{e^x + 1}:dx
end{equation}
Here let $u^2 = e^x + 1$. Employing Implicit Differentiation we see
begin{equation}
frac{d}{dx}left[u^2right] = frac{d}{dx}left[e^x + 1 right] rightarrow 2u frac{du}{dx} = e^x rightarrow dx = frac{2u}{e^x}:du = frac{2u}{u^2 - 1}:du
end{equation}
Thus:
begin{align}
I &= int e^x sqrt{e^x + 1}:dx = int left(u^2 - 1right)sqrt{u^2}frac{2u}{u^2 - 1}:du \
&= int 2u^2 :du = frac{u^3}{3} + C = frac{1}{3}left(e^x + 1 right)^frac{3}{2} + C
end{align}
Where $C$ is the constant of integration.
$endgroup$
Another approach:
begin{equation}
I = int e^x sqrt{e^x + 1}:dx
end{equation}
Here let $u^2 = e^x + 1$. Employing Implicit Differentiation we see
begin{equation}
frac{d}{dx}left[u^2right] = frac{d}{dx}left[e^x + 1 right] rightarrow 2u frac{du}{dx} = e^x rightarrow dx = frac{2u}{e^x}:du = frac{2u}{u^2 - 1}:du
end{equation}
Thus:
begin{align}
I &= int e^x sqrt{e^x + 1}:dx = int left(u^2 - 1right)sqrt{u^2}frac{2u}{u^2 - 1}:du \
&= int 2u^2 :du = frac{u^3}{3} + C = frac{1}{3}left(e^x + 1 right)^frac{3}{2} + C
end{align}
Where $C$ is the constant of integration.
answered Jan 13 at 12:49
user150203
add a comment |
add a comment |
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$begingroup$
You should not forget about writing $dx $ every time you deal with an integral.
$endgroup$
– user
Jan 10 at 20:46