If log 2, log


If log 2, log (2x − 1) and log (2x + 3) are in A.P., write the value of x.


The numbers $\log 2, \log \left(2^{x}-1\right)$ and $\log \left(2^{x}+3\right)$ are in A.P.

$\therefore \log \left(2^{x}-1\right)-\log 2=\log \left(2^{x}+3\right)-\log \left(2^{x}-1\right)$

$\Rightarrow \log \left(\frac{2^{x}-1}{2}\right)=\log \left(\frac{2^{x}+3}{2^{x}-1}\right)$

$\Rightarrow \frac{2^{x}-1}{2}=\frac{2^{x}+3}{2^{x}-1}$


$\Rightarrow 2^{2 x}+1-2.2^{x}=2.2^{x}+6$

$\Rightarrow 2^{2 x}-4.2^{x}-5=0$

Let $2^{x}=y$.

$\Rightarrow y^{2}-4 y-5=0$


$\Rightarrow y=5$ or $y=-1$

$\therefore 2^{x}=5$ or $2^{x}=-1$ (not possible)

Taking log on both the sides, we get:

$\log 2^{x}=\log 5$

$\Rightarrow x \log 2=\log 5$

$\Rightarrow x=\frac{\log 5}{\log 2}=\log _{2} 5$

$\Rightarrow x=\log _{2} 5$

Disclaimer: The question in the book has some error, so, this solution is not matching with the solution given in the book. This solution here is created according to the question given in the book.

Leave a comment


Click here to get exam-ready with eSaral

For making your preparation journey smoother of JEE, NEET and Class 8 to 10, grab our app now.

Download Now