Solving the equation $frac{ln (x)}{ln (1-x)} = frac{1}{x} - 1$
I'm trying to solve the following equation (which has solution $x = 1/2$)
$$frac{ln (x)}{ln (1-x)} = frac{1}{x} - 1 $$
I can't seem to do it analytically. Any ideas?
calculus algebra-precalculus logarithms
add a comment |
I'm trying to solve the following equation (which has solution $x = 1/2$)
$$frac{ln (x)}{ln (1-x)} = frac{1}{x} - 1 $$
I can't seem to do it analytically. Any ideas?
calculus algebra-precalculus logarithms
1
Sidenote: It can relatively easily be shown that this is equivalent to solving $x^x=left(1-xright)^{left(1-xright)}$ but I'm not sure if that leads anywhere.
– Jam
Dec 7 at 13:58
add a comment |
I'm trying to solve the following equation (which has solution $x = 1/2$)
$$frac{ln (x)}{ln (1-x)} = frac{1}{x} - 1 $$
I can't seem to do it analytically. Any ideas?
calculus algebra-precalculus logarithms
I'm trying to solve the following equation (which has solution $x = 1/2$)
$$frac{ln (x)}{ln (1-x)} = frac{1}{x} - 1 $$
I can't seem to do it analytically. Any ideas?
calculus algebra-precalculus logarithms
calculus algebra-precalculus logarithms
edited Dec 7 at 9:27
Martin Sleziak
44.6k7115270
44.6k7115270
asked Dec 7 at 8:09
user308485
49039
49039
1
Sidenote: It can relatively easily be shown that this is equivalent to solving $x^x=left(1-xright)^{left(1-xright)}$ but I'm not sure if that leads anywhere.
– Jam
Dec 7 at 13:58
add a comment |
1
Sidenote: It can relatively easily be shown that this is equivalent to solving $x^x=left(1-xright)^{left(1-xright)}$ but I'm not sure if that leads anywhere.
– Jam
Dec 7 at 13:58
1
1
Sidenote: It can relatively easily be shown that this is equivalent to solving $x^x=left(1-xright)^{left(1-xright)}$ but I'm not sure if that leads anywhere.
– Jam
Dec 7 at 13:58
Sidenote: It can relatively easily be shown that this is equivalent to solving $x^x=left(1-xright)^{left(1-xright)}$ but I'm not sure if that leads anywhere.
– Jam
Dec 7 at 13:58
add a comment |
3 Answers
3
active
oldest
votes
Let $f(x)=x ln x$. Then the given equation is
$$f(x)=f(1-x).$$
This is symmetric about $x=1/2$. Hence $x=1/2$ is a solution.
4
Very nice solution!
– gimusi
Dec 7 at 8:17
5
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
2
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
add a comment |
The function $xlog x-(1-x)log(1-x)$ is only defined in $(0,1)$, is differentiable, has a root at $x=frac12$ (by inspection) and tends to zero at the interval endpoints.
The derivative,
$$log x+log(1-x)+2$$ has two roots in $(0,1)$, where $x(1-x)=e^{-2}$.
Hence the function is negative in $(0,frac12)$, with a single minimum, and positive in $(frac12,1)$, with a single maximum, and there are no other roots.
add a comment |
Let $g:Itomathbb{R}$, where $I:=left[-dfrac12,+dfrac12right]$, be the function defined by $$g(t):=begin{cases}left(dfrac{1}{2}+tright),lnleft(dfrac12+tright)-left(dfrac{1}{2}-tright),lnleft(dfrac12-tright)&text{if }tinleft(-dfrac12,+dfrac12right),,\0&text{if }tinleft{-dfrac12,+dfrac12right},.end{cases}$$ (Observe that $g$ is continuous on the whole $I$, and is smooth on $left(-dfrac12,+dfrac12right)$.) Then, we are to solve for $xin(0,1)$ from $$gleft(x-dfrac12right)=0,,$$
which is equivalent to solving for $yin left(-dfrac12,+dfrac12right)$ such that
$$g(y)=0text{ by setting }y:=x-dfrac12,.$$
We claim that $g$ has only three roots $-dfrac12,0,+dfrac12$, and this means the only $yinleft(-dfrac12,+dfrac12right)$ such that $g(y)=0$ is $y=0$, making $x=dfrac12$ the sole solution to the required equation. Suppose contrary that $g$ has more than three roots. Then, by symmetry, on the interval $left[0,dfrac12right]$, $g$ has at least three roots. Using Rolle's Theorem, $g'$ has at least two roots on $left(0,dfrac12right)$, and so $g''$ has at least one root on $left(0,dfrac12right)$. However, we have
$$g'(t)=lnleft(frac12+tright)+lnleft(frac12-tright)+2,,$$
$$g''(t)=frac{1}{frac{1}{2}+t}-frac{1}{frac{1}{2}-t},,$$
and
$$g'''(t)=-frac{1}{left(frac{1}{2}+tright)^2}-frac{1}{left(frac12-tright)^2}<0$$
for all $tinleft(0,dfrac12right)$, which is a contradiction we seek.
add a comment |
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3 Answers
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3 Answers
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Let $f(x)=x ln x$. Then the given equation is
$$f(x)=f(1-x).$$
This is symmetric about $x=1/2$. Hence $x=1/2$ is a solution.
4
Very nice solution!
– gimusi
Dec 7 at 8:17
5
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
2
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
add a comment |
Let $f(x)=x ln x$. Then the given equation is
$$f(x)=f(1-x).$$
This is symmetric about $x=1/2$. Hence $x=1/2$ is a solution.
4
Very nice solution!
– gimusi
Dec 7 at 8:17
5
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
2
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
add a comment |
Let $f(x)=x ln x$. Then the given equation is
$$f(x)=f(1-x).$$
This is symmetric about $x=1/2$. Hence $x=1/2$ is a solution.
Let $f(x)=x ln x$. Then the given equation is
$$f(x)=f(1-x).$$
This is symmetric about $x=1/2$. Hence $x=1/2$ is a solution.
answered Dec 7 at 8:11
Anurag A
25.5k12249
25.5k12249
4
Very nice solution!
– gimusi
Dec 7 at 8:17
5
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
2
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
add a comment |
4
Very nice solution!
– gimusi
Dec 7 at 8:17
5
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
2
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
4
4
Very nice solution!
– gimusi
Dec 7 at 8:17
Very nice solution!
– gimusi
Dec 7 at 8:17
5
5
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
I think this is insufficient. Proving that $x=dfrac12$ is a solution is easy. Proving that it is the only solution is not as easy, but I can see that one may approach it by showing that $$f(x)-f(1-x)<0text{ for all }xtext{ such that }0<x<frac12,,$$ and by symmetry $$f(x)-f(1-x)>0text{ for all }xtext{ such that }frac12<x<1,.$$
– Batominovski
Dec 7 at 8:41
2
2
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
Yep, IMO too many upvotes as the existence of other solutions should be discussed.
– Yves Daoust
Dec 7 at 12:53
add a comment |
The function $xlog x-(1-x)log(1-x)$ is only defined in $(0,1)$, is differentiable, has a root at $x=frac12$ (by inspection) and tends to zero at the interval endpoints.
The derivative,
$$log x+log(1-x)+2$$ has two roots in $(0,1)$, where $x(1-x)=e^{-2}$.
Hence the function is negative in $(0,frac12)$, with a single minimum, and positive in $(frac12,1)$, with a single maximum, and there are no other roots.
add a comment |
The function $xlog x-(1-x)log(1-x)$ is only defined in $(0,1)$, is differentiable, has a root at $x=frac12$ (by inspection) and tends to zero at the interval endpoints.
The derivative,
$$log x+log(1-x)+2$$ has two roots in $(0,1)$, where $x(1-x)=e^{-2}$.
Hence the function is negative in $(0,frac12)$, with a single minimum, and positive in $(frac12,1)$, with a single maximum, and there are no other roots.
add a comment |
The function $xlog x-(1-x)log(1-x)$ is only defined in $(0,1)$, is differentiable, has a root at $x=frac12$ (by inspection) and tends to zero at the interval endpoints.
The derivative,
$$log x+log(1-x)+2$$ has two roots in $(0,1)$, where $x(1-x)=e^{-2}$.
Hence the function is negative in $(0,frac12)$, with a single minimum, and positive in $(frac12,1)$, with a single maximum, and there are no other roots.
The function $xlog x-(1-x)log(1-x)$ is only defined in $(0,1)$, is differentiable, has a root at $x=frac12$ (by inspection) and tends to zero at the interval endpoints.
The derivative,
$$log x+log(1-x)+2$$ has two roots in $(0,1)$, where $x(1-x)=e^{-2}$.
Hence the function is negative in $(0,frac12)$, with a single minimum, and positive in $(frac12,1)$, with a single maximum, and there are no other roots.
edited Dec 7 at 13:17
answered Dec 7 at 13:10
Yves Daoust
124k671221
124k671221
add a comment |
add a comment |
Let $g:Itomathbb{R}$, where $I:=left[-dfrac12,+dfrac12right]$, be the function defined by $$g(t):=begin{cases}left(dfrac{1}{2}+tright),lnleft(dfrac12+tright)-left(dfrac{1}{2}-tright),lnleft(dfrac12-tright)&text{if }tinleft(-dfrac12,+dfrac12right),,\0&text{if }tinleft{-dfrac12,+dfrac12right},.end{cases}$$ (Observe that $g$ is continuous on the whole $I$, and is smooth on $left(-dfrac12,+dfrac12right)$.) Then, we are to solve for $xin(0,1)$ from $$gleft(x-dfrac12right)=0,,$$
which is equivalent to solving for $yin left(-dfrac12,+dfrac12right)$ such that
$$g(y)=0text{ by setting }y:=x-dfrac12,.$$
We claim that $g$ has only three roots $-dfrac12,0,+dfrac12$, and this means the only $yinleft(-dfrac12,+dfrac12right)$ such that $g(y)=0$ is $y=0$, making $x=dfrac12$ the sole solution to the required equation. Suppose contrary that $g$ has more than three roots. Then, by symmetry, on the interval $left[0,dfrac12right]$, $g$ has at least three roots. Using Rolle's Theorem, $g'$ has at least two roots on $left(0,dfrac12right)$, and so $g''$ has at least one root on $left(0,dfrac12right)$. However, we have
$$g'(t)=lnleft(frac12+tright)+lnleft(frac12-tright)+2,,$$
$$g''(t)=frac{1}{frac{1}{2}+t}-frac{1}{frac{1}{2}-t},,$$
and
$$g'''(t)=-frac{1}{left(frac{1}{2}+tright)^2}-frac{1}{left(frac12-tright)^2}<0$$
for all $tinleft(0,dfrac12right)$, which is a contradiction we seek.
add a comment |
Let $g:Itomathbb{R}$, where $I:=left[-dfrac12,+dfrac12right]$, be the function defined by $$g(t):=begin{cases}left(dfrac{1}{2}+tright),lnleft(dfrac12+tright)-left(dfrac{1}{2}-tright),lnleft(dfrac12-tright)&text{if }tinleft(-dfrac12,+dfrac12right),,\0&text{if }tinleft{-dfrac12,+dfrac12right},.end{cases}$$ (Observe that $g$ is continuous on the whole $I$, and is smooth on $left(-dfrac12,+dfrac12right)$.) Then, we are to solve for $xin(0,1)$ from $$gleft(x-dfrac12right)=0,,$$
which is equivalent to solving for $yin left(-dfrac12,+dfrac12right)$ such that
$$g(y)=0text{ by setting }y:=x-dfrac12,.$$
We claim that $g$ has only three roots $-dfrac12,0,+dfrac12$, and this means the only $yinleft(-dfrac12,+dfrac12right)$ such that $g(y)=0$ is $y=0$, making $x=dfrac12$ the sole solution to the required equation. Suppose contrary that $g$ has more than three roots. Then, by symmetry, on the interval $left[0,dfrac12right]$, $g$ has at least three roots. Using Rolle's Theorem, $g'$ has at least two roots on $left(0,dfrac12right)$, and so $g''$ has at least one root on $left(0,dfrac12right)$. However, we have
$$g'(t)=lnleft(frac12+tright)+lnleft(frac12-tright)+2,,$$
$$g''(t)=frac{1}{frac{1}{2}+t}-frac{1}{frac{1}{2}-t},,$$
and
$$g'''(t)=-frac{1}{left(frac{1}{2}+tright)^2}-frac{1}{left(frac12-tright)^2}<0$$
for all $tinleft(0,dfrac12right)$, which is a contradiction we seek.
add a comment |
Let $g:Itomathbb{R}$, where $I:=left[-dfrac12,+dfrac12right]$, be the function defined by $$g(t):=begin{cases}left(dfrac{1}{2}+tright),lnleft(dfrac12+tright)-left(dfrac{1}{2}-tright),lnleft(dfrac12-tright)&text{if }tinleft(-dfrac12,+dfrac12right),,\0&text{if }tinleft{-dfrac12,+dfrac12right},.end{cases}$$ (Observe that $g$ is continuous on the whole $I$, and is smooth on $left(-dfrac12,+dfrac12right)$.) Then, we are to solve for $xin(0,1)$ from $$gleft(x-dfrac12right)=0,,$$
which is equivalent to solving for $yin left(-dfrac12,+dfrac12right)$ such that
$$g(y)=0text{ by setting }y:=x-dfrac12,.$$
We claim that $g$ has only three roots $-dfrac12,0,+dfrac12$, and this means the only $yinleft(-dfrac12,+dfrac12right)$ such that $g(y)=0$ is $y=0$, making $x=dfrac12$ the sole solution to the required equation. Suppose contrary that $g$ has more than three roots. Then, by symmetry, on the interval $left[0,dfrac12right]$, $g$ has at least three roots. Using Rolle's Theorem, $g'$ has at least two roots on $left(0,dfrac12right)$, and so $g''$ has at least one root on $left(0,dfrac12right)$. However, we have
$$g'(t)=lnleft(frac12+tright)+lnleft(frac12-tright)+2,,$$
$$g''(t)=frac{1}{frac{1}{2}+t}-frac{1}{frac{1}{2}-t},,$$
and
$$g'''(t)=-frac{1}{left(frac{1}{2}+tright)^2}-frac{1}{left(frac12-tright)^2}<0$$
for all $tinleft(0,dfrac12right)$, which is a contradiction we seek.
Let $g:Itomathbb{R}$, where $I:=left[-dfrac12,+dfrac12right]$, be the function defined by $$g(t):=begin{cases}left(dfrac{1}{2}+tright),lnleft(dfrac12+tright)-left(dfrac{1}{2}-tright),lnleft(dfrac12-tright)&text{if }tinleft(-dfrac12,+dfrac12right),,\0&text{if }tinleft{-dfrac12,+dfrac12right},.end{cases}$$ (Observe that $g$ is continuous on the whole $I$, and is smooth on $left(-dfrac12,+dfrac12right)$.) Then, we are to solve for $xin(0,1)$ from $$gleft(x-dfrac12right)=0,,$$
which is equivalent to solving for $yin left(-dfrac12,+dfrac12right)$ such that
$$g(y)=0text{ by setting }y:=x-dfrac12,.$$
We claim that $g$ has only three roots $-dfrac12,0,+dfrac12$, and this means the only $yinleft(-dfrac12,+dfrac12right)$ such that $g(y)=0$ is $y=0$, making $x=dfrac12$ the sole solution to the required equation. Suppose contrary that $g$ has more than three roots. Then, by symmetry, on the interval $left[0,dfrac12right]$, $g$ has at least three roots. Using Rolle's Theorem, $g'$ has at least two roots on $left(0,dfrac12right)$, and so $g''$ has at least one root on $left(0,dfrac12right)$. However, we have
$$g'(t)=lnleft(frac12+tright)+lnleft(frac12-tright)+2,,$$
$$g''(t)=frac{1}{frac{1}{2}+t}-frac{1}{frac{1}{2}-t},,$$
and
$$g'''(t)=-frac{1}{left(frac{1}{2}+tright)^2}-frac{1}{left(frac12-tright)^2}<0$$
for all $tinleft(0,dfrac12right)$, which is a contradiction we seek.
answered Dec 7 at 9:28
Batominovski
33.7k33292
33.7k33292
add a comment |
add a comment |
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1
Sidenote: It can relatively easily be shown that this is equivalent to solving $x^x=left(1-xright)^{left(1-xright)}$ but I'm not sure if that leads anywhere.
– Jam
Dec 7 at 13:58