Problem For Year 2029

Algebra Level 5

If x > y > 1 x>y>1 and 1 log x y + 1 log y x = 2029 \dfrac{1}{\log_xy}+\dfrac{1}{\log_yx}=\sqrt{2029} , find the value of 1 log x y x 1 log x y y \dfrac{1}{\log_{xy}x}-\dfrac{1}{\log_{xy}y} .


The answer is -45.

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1 solution

Rishabh Jain
Jul 25, 2016

First simplify few things...

( 1 ) (1) Recall log a b = 1 log b a \log_a b=\dfrac{1}{\log_b a}

& ( 2 ) (2) Substitute log y x = q \log_y x=q .

The given equation is:

log y x + 1 log y x q + 1 q = 2029 \large \underbrace{\log_y x+\dfrac{1}{\log_y x}}_{\large q+\frac 1q}=\sqrt{2029}

And required expression is :

log x x y log y x y = ( 1 + log x y ) ( 1 + log y x ) \log_x xy-\log_y xy=(\cancel{1}+\log_x y)-(\cancel{1}+\log_y x)

which is same as log x y log y x = 1 q q \log_x y-\log_y x=\dfrac 1q-q . Hence the question translates to:

Given : q + 1 q = 2029 , 1 q q = ? \text{Given}: q+\dfrac 1q=\sqrt{2029},\dfrac 1q-q=\ ?

Now we know:

1 q q = ( q + 1 q ) 2 4 = ( 2029 ) 4 = 45 \dfrac 1q-q=-\sqrt{\left(q+\dfrac 1q\right)^2-4}=-\sqrt{(2029)-4}=-\boxed{45}


NOTE:- Since x > y x>y ,

log y x > 1 ( log y x ) 2 > 1 q 2 > 1 q > 1 q 1 q q < 0 \log_y x>1\implies (\log_y x)^2>1\implies q^2>1\implies q>\frac 1q\implies \frac 1q-q<0

Check the answer please, I think it's 45 \boxed{ -45}

Guillermo Templado - 4 years, 10 months ago

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So sorry for that.... It was too late when I typed the solution and felt sleepy so I thought to edit the solution in morning so now I have edited it.

Rishabh Jain - 4 years, 10 months ago

The answer is definitely -45. Note how the first fraction has a larger denominator, and thus is smaller, than the second one. Which is why the answer should be negative.

Alon Heller - 4 years, 10 months ago

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So sorry for that.... It was too late when I typed the solution and felt sleepy so I thought to edit the solution in morning so now I have edited it.

Rishabh Jain - 4 years, 10 months ago

Nice question

Jaya Gupta - 4 years, 10 months ago

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