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Algebra Level 4

log 2 ( 1 + 4 x ) + log 2 ( 1 4 x + 4 ) = 2 log 2 ( 2 x 1 1 ) \large\log^2\left(1+\dfrac 4x\right)+\log^2\left(1-\dfrac{4}{x+4}\right)=2\log^2\left(\dfrac{2}{x-1}-1\right)

How many real solutions does the above equation have?

2 0 4 Infinitely many

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1 solution

Rishabh Jain
Apr 7, 2016

Some simplification gives: 2 log 2 ( x + 4 x ) = 2 log 2 ( 3 x x 1 ) ( ) 2\log^2\left(\dfrac{x+4}{x}\right)=2\log^2\left(\dfrac{3-x}{x-1}\right)~(**) log ( x + 4 x ) = ± log ( 3 x x 1 ) = log ( 3 x x 1 ) ± 1 \implies \log \left(\dfrac{x+4}{x}\right)=\pm\log\left(\dfrac{3-x}{x-1}\right)= \log\left(\dfrac{3-x}{x-1}\right)^{\pm 1} x + 4 x = 3 x x 1 and x + 4 x = x 1 3 x \implies \dfrac{x+4}{x}= \dfrac{3-x}{x-1} \text{ and }\dfrac{x+4}{x}=\dfrac{x-1}{3-x}

x 2 2 = 0 and x 2 6 = 0 \implies x^2-2=0\text{ and }x^2-6=0

x = 2 , 6 ( 1 , 3 ) (See below) \implies \large x=\sqrt 2,\sqrt 6\in(1,3)\small{\color{#3D99F6}{\text{ (See below) }}} i.e 2 \large \boxed 2 values. \therefore the correct option is x 2 7 x + 10 = 0 x^2-7x+10=0 since 2 2 is a root of this equation only.


Since domain of log \log function is R + \mathbb{R^+} we get x + 4 x > 0 \dfrac{x+4}{x}>0 and 3 x x 1 > 0 \dfrac{3-x}{x-1}>0 . Solving we get x ( 1 , 3 ) x\in(1,3) .

@Rishabh Cool B. how much marks are you getting in mains??? and your classmates.....my analysis is this year paper was quite on a simpler side and easy as comp. to last year....how was your paper

Mohit Gupta - 5 years, 2 months ago

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I got that right. Yahooo!!!!!!

Sai Ram - 5 years, 2 months ago

Why did you delete your comment?

Manish Maharaj - 5 years, 2 months ago

Stunniing solution, nice work!

Ashish Menon - 4 years, 11 months ago

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Thanks.... :-)

Rishabh Jain - 4 years, 11 months ago

Isn't 1-(4/(x+4)) equal to x/(x+4), not (x+4)/x? Then wouldn't the first step in the solution be incorrect?

Thomas Horstkamp - 5 years, 2 months ago

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log ( x x + 4 ) \log\left(\dfrac{x}{x+4}\right) = log ( x + 4 x ) 1 =\log\left(\dfrac{x+4}{x}\right)^{-1} = log ( x + 4 x ) =-\log\left(\dfrac{x+4}{x}\right)

And minus sign is removed when we square it to get : log 2 ( x + 4 x ) \log^2\left(\dfrac{x+4}{x}\right)

Rishabh Jain - 5 years, 2 months ago

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Thanks for the clarification. That problem was cleverly made.

Thomas Horstkamp - 5 years, 2 months ago

Shit made a mistake at the end rest was correct.

Pawan pal - 5 years, 1 month ago

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