The function $g(x)=3x+ln2x$ for $x>0$. Find $g'(x)$ and prove that $g(x)$ has an inverse function.
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The function $g(x)=3x+ln2x$ for $x>0$. Find $g'(x)$ and prove that $g(x)$ has an inverse function.
So $g'(x)=3+frac{1}{x} $ for $x>0$ But I have no idea how I'm supposed to prove that this means $g(x)$ has an inverse function. Any help will be appreciated
functions derivatives inverse-function
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add a comment |
$begingroup$
The function $g(x)=3x+ln2x$ for $x>0$. Find $g'(x)$ and prove that $g(x)$ has an inverse function.
So $g'(x)=3+frac{1}{x} $ for $x>0$ But I have no idea how I'm supposed to prove that this means $g(x)$ has an inverse function. Any help will be appreciated
functions derivatives inverse-function
$endgroup$
add a comment |
$begingroup$
The function $g(x)=3x+ln2x$ for $x>0$. Find $g'(x)$ and prove that $g(x)$ has an inverse function.
So $g'(x)=3+frac{1}{x} $ for $x>0$ But I have no idea how I'm supposed to prove that this means $g(x)$ has an inverse function. Any help will be appreciated
functions derivatives inverse-function
$endgroup$
The function $g(x)=3x+ln2x$ for $x>0$. Find $g'(x)$ and prove that $g(x)$ has an inverse function.
So $g'(x)=3+frac{1}{x} $ for $x>0$ But I have no idea how I'm supposed to prove that this means $g(x)$ has an inverse function. Any help will be appreciated
functions derivatives inverse-function
functions derivatives inverse-function
asked Dec 8 '18 at 21:38
H.LinkhornH.Linkhorn
401113
401113
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4 Answers
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$begingroup$
Hint: It follows from what you did that $x>0implies g'(x)>0$.
$endgroup$
add a comment |
$begingroup$
What is required for a function to be invertible? It must be one-to-one and onto. Your function is $g:mathbb{R}^+rightarrowmathbb{R}$. Establishing one-to-one-ness should be doable via the derivative you calculated, and establishing onto-ness should follow from continuity, the intermediate value theorem, and the appropriate limits to $0^+$ and $infty$.
$endgroup$
add a comment |
$begingroup$
Since you have the condition $x>0$ and $g'(x) > 0$ for positive $x$, then you know $g$ is strictly increasing. This means that $g$ is injective on $(0,infty)$ and has an inverse on that interval.
$endgroup$
add a comment |
$begingroup$
Generally speaking, there are two `standard' ways to show the existence of something. The first is to construct it, and the second is to show that if we suppose that such a thing does not exist, we get a contradiction. The intuitionist in me would always recommend the former before falling back on the latter. In this case you need to show that $g'(x)=3+frac{1}{x}$. To find the inverse of a function $f$, I was taught to substitute to let $y=f(x)$, swap the roles of $x$ and $y$, then rearrange for $y$. In this case, this amounts to setting $x=g'(y)=3+frac{1}{y}$ and rearranging for $y$. Good luck!
EDIT:
It's been pointed out that you've been asked to find the inverse of $g$ and not of $g'$. In that case, we recall the following lemma:
Let $f:Ito mathbb{R}$ be a strictly monotonic, continuous function on an interval $I$. Then $f$ has an inverse.
In light of this lemma, it suffices to show that $g$ is monotonic (since $mathbb{R}^+=(0,infty)$ is an interval). To do so we require that $g'(x)>0$ (or $g'(x)<0$) $forall xin mathbb{R}^+$. The result follows quickly from you calculation.
$endgroup$
1
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
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
$begingroup$
Hint: It follows from what you did that $x>0implies g'(x)>0$.
$endgroup$
add a comment |
$begingroup$
Hint: It follows from what you did that $x>0implies g'(x)>0$.
$endgroup$
add a comment |
$begingroup$
Hint: It follows from what you did that $x>0implies g'(x)>0$.
$endgroup$
Hint: It follows from what you did that $x>0implies g'(x)>0$.
answered Dec 8 '18 at 21:40
José Carlos SantosJosé Carlos Santos
160k22127232
160k22127232
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$begingroup$
What is required for a function to be invertible? It must be one-to-one and onto. Your function is $g:mathbb{R}^+rightarrowmathbb{R}$. Establishing one-to-one-ness should be doable via the derivative you calculated, and establishing onto-ness should follow from continuity, the intermediate value theorem, and the appropriate limits to $0^+$ and $infty$.
$endgroup$
add a comment |
$begingroup$
What is required for a function to be invertible? It must be one-to-one and onto. Your function is $g:mathbb{R}^+rightarrowmathbb{R}$. Establishing one-to-one-ness should be doable via the derivative you calculated, and establishing onto-ness should follow from continuity, the intermediate value theorem, and the appropriate limits to $0^+$ and $infty$.
$endgroup$
add a comment |
$begingroup$
What is required for a function to be invertible? It must be one-to-one and onto. Your function is $g:mathbb{R}^+rightarrowmathbb{R}$. Establishing one-to-one-ness should be doable via the derivative you calculated, and establishing onto-ness should follow from continuity, the intermediate value theorem, and the appropriate limits to $0^+$ and $infty$.
$endgroup$
What is required for a function to be invertible? It must be one-to-one and onto. Your function is $g:mathbb{R}^+rightarrowmathbb{R}$. Establishing one-to-one-ness should be doable via the derivative you calculated, and establishing onto-ness should follow from continuity, the intermediate value theorem, and the appropriate limits to $0^+$ and $infty$.
answered Dec 8 '18 at 21:41
RandomMathGuyRandomMathGuy
462
462
add a comment |
add a comment |
$begingroup$
Since you have the condition $x>0$ and $g'(x) > 0$ for positive $x$, then you know $g$ is strictly increasing. This means that $g$ is injective on $(0,infty)$ and has an inverse on that interval.
$endgroup$
add a comment |
$begingroup$
Since you have the condition $x>0$ and $g'(x) > 0$ for positive $x$, then you know $g$ is strictly increasing. This means that $g$ is injective on $(0,infty)$ and has an inverse on that interval.
$endgroup$
add a comment |
$begingroup$
Since you have the condition $x>0$ and $g'(x) > 0$ for positive $x$, then you know $g$ is strictly increasing. This means that $g$ is injective on $(0,infty)$ and has an inverse on that interval.
$endgroup$
Since you have the condition $x>0$ and $g'(x) > 0$ for positive $x$, then you know $g$ is strictly increasing. This means that $g$ is injective on $(0,infty)$ and has an inverse on that interval.
answered Dec 8 '18 at 21:48
Dando18Dando18
4,67741235
4,67741235
add a comment |
add a comment |
$begingroup$
Generally speaking, there are two `standard' ways to show the existence of something. The first is to construct it, and the second is to show that if we suppose that such a thing does not exist, we get a contradiction. The intuitionist in me would always recommend the former before falling back on the latter. In this case you need to show that $g'(x)=3+frac{1}{x}$. To find the inverse of a function $f$, I was taught to substitute to let $y=f(x)$, swap the roles of $x$ and $y$, then rearrange for $y$. In this case, this amounts to setting $x=g'(y)=3+frac{1}{y}$ and rearranging for $y$. Good luck!
EDIT:
It's been pointed out that you've been asked to find the inverse of $g$ and not of $g'$. In that case, we recall the following lemma:
Let $f:Ito mathbb{R}$ be a strictly monotonic, continuous function on an interval $I$. Then $f$ has an inverse.
In light of this lemma, it suffices to show that $g$ is monotonic (since $mathbb{R}^+=(0,infty)$ is an interval). To do so we require that $g'(x)>0$ (or $g'(x)<0$) $forall xin mathbb{R}^+$. The result follows quickly from you calculation.
$endgroup$
1
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
add a comment |
$begingroup$
Generally speaking, there are two `standard' ways to show the existence of something. The first is to construct it, and the second is to show that if we suppose that such a thing does not exist, we get a contradiction. The intuitionist in me would always recommend the former before falling back on the latter. In this case you need to show that $g'(x)=3+frac{1}{x}$. To find the inverse of a function $f$, I was taught to substitute to let $y=f(x)$, swap the roles of $x$ and $y$, then rearrange for $y$. In this case, this amounts to setting $x=g'(y)=3+frac{1}{y}$ and rearranging for $y$. Good luck!
EDIT:
It's been pointed out that you've been asked to find the inverse of $g$ and not of $g'$. In that case, we recall the following lemma:
Let $f:Ito mathbb{R}$ be a strictly monotonic, continuous function on an interval $I$. Then $f$ has an inverse.
In light of this lemma, it suffices to show that $g$ is monotonic (since $mathbb{R}^+=(0,infty)$ is an interval). To do so we require that $g'(x)>0$ (or $g'(x)<0$) $forall xin mathbb{R}^+$. The result follows quickly from you calculation.
$endgroup$
1
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
add a comment |
$begingroup$
Generally speaking, there are two `standard' ways to show the existence of something. The first is to construct it, and the second is to show that if we suppose that such a thing does not exist, we get a contradiction. The intuitionist in me would always recommend the former before falling back on the latter. In this case you need to show that $g'(x)=3+frac{1}{x}$. To find the inverse of a function $f$, I was taught to substitute to let $y=f(x)$, swap the roles of $x$ and $y$, then rearrange for $y$. In this case, this amounts to setting $x=g'(y)=3+frac{1}{y}$ and rearranging for $y$. Good luck!
EDIT:
It's been pointed out that you've been asked to find the inverse of $g$ and not of $g'$. In that case, we recall the following lemma:
Let $f:Ito mathbb{R}$ be a strictly monotonic, continuous function on an interval $I$. Then $f$ has an inverse.
In light of this lemma, it suffices to show that $g$ is monotonic (since $mathbb{R}^+=(0,infty)$ is an interval). To do so we require that $g'(x)>0$ (or $g'(x)<0$) $forall xin mathbb{R}^+$. The result follows quickly from you calculation.
$endgroup$
Generally speaking, there are two `standard' ways to show the existence of something. The first is to construct it, and the second is to show that if we suppose that such a thing does not exist, we get a contradiction. The intuitionist in me would always recommend the former before falling back on the latter. In this case you need to show that $g'(x)=3+frac{1}{x}$. To find the inverse of a function $f$, I was taught to substitute to let $y=f(x)$, swap the roles of $x$ and $y$, then rearrange for $y$. In this case, this amounts to setting $x=g'(y)=3+frac{1}{y}$ and rearranging for $y$. Good luck!
EDIT:
It's been pointed out that you've been asked to find the inverse of $g$ and not of $g'$. In that case, we recall the following lemma:
Let $f:Ito mathbb{R}$ be a strictly monotonic, continuous function on an interval $I$. Then $f$ has an inverse.
In light of this lemma, it suffices to show that $g$ is monotonic (since $mathbb{R}^+=(0,infty)$ is an interval). To do so we require that $g'(x)>0$ (or $g'(x)<0$) $forall xin mathbb{R}^+$. The result follows quickly from you calculation.
edited Dec 8 '18 at 22:19
answered Dec 8 '18 at 22:01
BenBen
1499
1499
1
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
add a comment |
1
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
1
1
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
I don't believe they're supposed to find $(g')^{-1}$, but rather $g^{-1}$.
$endgroup$
– Dando18
Dec 8 '18 at 22:06
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Just re-read, thanks for setting me straight
$endgroup$
– Ben
Dec 8 '18 at 22:08
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
Additionally, computing the inverse or contradicting that $g$ has no inverse seems like overkill here when we know that any strictly monotonic function is bijective and therefore has an inverse.
$endgroup$
– Dando18
Dec 8 '18 at 22:10
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
$begingroup$
I've added an edit, as I said those are standard ways of approaching any construction proof. Of course, as you say, we actually have a direct approach in this situation.
$endgroup$
– Ben
Dec 8 '18 at 22:20
add a comment |
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