A tower system

Algebra Level 4

x y x y x y . . = 2 , y x y x y x . . = 3 2 \huge x^{y^{x^{y^{x^{y^{.^{.}}}}}}} = 2 \quad,\quad y^{x^{y^{x^{y^{x^{.^{.}}}}}}} = \frac{3}{2}

Let x x and y y be real numbers such that they satisfy both the equations above. If x y xy can be expressed in the form a b n \sqrt[n]{\frac{a}{b}} where g c d ( a , b ) = 1 gcd(a,b)=1 and n n is the minimum positive integer satisfying the expression, determine

L C M ( a , b , n ) ( a + b + n ) LCM(a,b,n) - (a+b+n)


The answer is -7.

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3 solutions

Sharky Kesa
Oct 27, 2015

Since x y x = 2 x^{y^{x^\ldots}} = 2 and y x y = 3 2 y^{x^{y^\ldots}}=\dfrac{3}{2} , we have

x 3 2 = 2 x^{\frac{3}{2}} = 2

y 2 = 3 2 y^{2} = \frac{3}{2}

by substitution. Simplifying and converting, we have

x = 2 2 3 = 2 4 6 = 16 6 x = 2^{\frac{2}{3}} = 2^{\frac{4}{6}} = \sqrt[6]{16}

y = ( 3 2 ) 1 2 = ( 3 2 ) 3 6 = 27 8 6 y=\left (\dfrac{3}{2} \right )^{\frac{1}{2}} = \left (\dfrac{3}{2} \right )^{\frac{3}{6}} = \sqrt[6]{\dfrac{27}{8}}

Therefore,

x y = 16 6 × 27 8 6 = 54 6 xy = \sqrt[6]{16} \times \sqrt[6]{\dfrac{27}{8}} = \sqrt[6]{54}

Hence, a = 54 , b = 1 , c = 6 a=54, b=1, c=6 .

L C M ( 54 , 1 , 6 ) = 54 LCM(54, 1, 6) = 54

54 ( 54 + 6 + 1 ) = 7 54-(54+6+1) = -7

Therefore, the answer is 7 -7 .


The reason I converted the fractions in the exponents into like fractions is so that it would be easy to multiply out the expression in the end.

Great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great great. Question: how many vowels in my reply? Anyway, yours is a very nice solution.

Debmeet Banerjee - 5 years, 7 months ago

The question refers that gcd of a and be is 1 which is meaningless coz it cannot be expressed in rational form

Aditya Narayan Sharma - 5 years, 6 months ago

Very nice solution..

Rakshit Joshi - 5 years, 7 months ago

But you have posted your final answer as 7 \boxed{7}

Atul Shivam - 5 years, 7 months ago

Same solution here bro :D (y)

A Former Brilliant Member - 5 years, 7 months ago
Atul Shivam
Oct 30, 2015

x 3 2 = 2 x^\frac{3}{2}=2 and y 2 = 3 2 y^2=\frac{3}{2} upon simplifying x × y x×y we will have 54 6 = x y \sqrt[6]{54}=xy

Now a = 54 , b = 1 , c = 6 a=54,b=1,c=6 L . C . M ( 54 , 1 , 6 ) = 54 L.C.M (54,1,6)=54

Now simply calculate 54 ( 54 + 6 + 1 ) 54-(54+6+1) which is 7 \boxed{-7}

Same solution lang kami ni Kesa

Haha, huwag ka magtagalog bro, hindi naman yata Pinoy karamihan dito eh :D

Efren Medallo - 5 years, 7 months ago

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Should i speak bisaya? hahaha xD

A Former Brilliant Member - 5 years, 7 months ago

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Hahahaha! Eh pareho lang namang hindi maiintindihan yun ng iba eh XD

Anyway, it feels good to see a fellow Pinoy in this site :)

Efren Medallo - 5 years, 7 months ago

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