Shame on me,
Pranav! It appears that we have found the same answer, which I'm not completely sure is correct...at first I solved it and got the solution set for $k$ to be $[-\dfrac{1}{2},0]$. And I was so happy, but I didn't check to see if it's correct or not!
Now, after reading posts from
Opalg and
kaliprasad, I checked my work again, and I found the same answer as you. But, I must also tell you I am not entirely sure if ${k: -\infty<k \le -\dfrac{1}{2}}$ is the answer because to me,
Opalg is always right.
I apologize for that.
Hi
Opalg,
Thanks for participating!:)
But
Opalg, my answer is different than yours. Could you please check my work?
My solution:
First off, it's very important and crucial to always find the domain of the original given logarithmic equation and in this problem, we have
[TABLE="class: grid, width: 600"]
[TR]
[TD]$kx+1>0\;\;\;$[/TD]
[TD]$2-x>0\;\;\;$
$-x>-2$
$x<2$[/TD]
[TD]$x-k>0\;\;\;$
$x>k$[/TD]
[TD]That is, the solution sets that we have for $x$ and $k$ must satisfy these three inequalities simultaneously.[/TD]
[/TR]
[/TABLE]
Also, the original equation $\log (kx+1)-\log (x-k)=\log (2-x) \rightarrow \log (kx+1)=\log ((2-x)(x-k))$ is equivalent to the quadratic equation $kx+1=(2-x)(x-k) \rightarrow x^2-2x+2k+1=0$ after we equate the arguments of the log functions.
The quadratic equation $x^2-2x+2k+1=0$ has the two solutions, namely $x=1+\sqrt{-2k}$ and $x=1-\sqrt{-2k}$.
At this point, we can conclude that
1. If $k=0$, we achieved what we wanted and $k=0$ is a solution to the problem,
2. $k$ cannot be greater than zero,
2. We need to think also the case when $k<0$.
Bearing in mind the restriction given by the problem is the given equation has a unique real solution, we thus consider
[TABLE="class: grid, width: 600"]
[TR]
[TD]$k<0$, $x>k$ and $x=1-\sqrt{-2k}$
[/TD]
[TD]and[/TD]
[TD]$k<0$, $x<2$ and $x=1+\sqrt{-2k}$[/TD]
[/TR]
[/TABLE]
[TABLE="class: grid, width: 500"]
[TR]
[TD]If $1-\sqrt{-2k}>k$ is true,[/TD]
[TD]then[/TD]
[TD]$1+\sqrt{-2k}>2$ must hold.[/TD]
[/TR]
[/TABLE]
If we play with the inequality below a bit, we see that
$1-\sqrt{-2k}>k$
$1-k>\sqrt{-2k}$
$(1-k)^2>(\sqrt{-2k})^2$
$1-2k+k^2>-2k$
$1+k^2>0$
That is, $1-\sqrt{-2k}>k$ is always true for all $k \in R$.
In this case, in order to find the solution set for $k$, which is the answer to the problem, we must set
$1+\sqrt{-2k}>2$
$\sqrt{-2k}>1$
$-2k>1$
$k<-\frac{1}{2}$
Hence, the solution set for $k$ is ${k:k<-\frac{1}{2}, k=0}$.
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Hey
kaliprasad, yes, if we approach it like you did, things can become excessively complicated...
BTW, thanks for participating!