Logarithms!

Algebra Level 4

log 2 x + log 4 y + log 4 z = 2 log 3 y + log 9 z + log 9 x = 2 log 4 z + log 16 x + log 16 y = 2 \large \begin{aligned} \log_2 x + \log_4 y + \log_4 z &=& 2 \\ \log_3 y + \log_9 z + \log_9 x &=& 2 \\ \log_4 z + \log_{16} x + \log_{16} y &=& 2 \end{aligned}

Solve the system equations where x x , y y and z z are positive real numbers.

If the value x , y , z x,y,z can be written in the form a x b x , a y b y , a z b z \dfrac{a_x}{b_x} , \dfrac{a_y}{b_y} , \dfrac{a_z}{b_z} for co-prime a i , a j a_i , a_j , find the value of a x + b x + a y + b y + a z + b z a_x+b_x+a_y+b_y+a_z+b_z .


The answer is 75.

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2 solutions

Personal Data
Jun 7, 2015

We can rewrite the equations as following

log 2 x + log 4 y + log 4 z = log 4 x 2 y z = 2 x 2 y z = 2 4 \log _{ 2 }{ x } +\log _{ 4 }{ y } +\log _{ 4 }{ z } =\log _{ 4 }{ { x }^{ 2 }yz } =2\Rightarrow { x }^{ 2 }yz={ 2 }^{ 4 }

log 3 y + log 9 z + log 9 x = log 9 y 2 z x = 2 y 2 z x = 3 4 \log _{ 3 }{ y } +\log _{ 9 }{ z } +\log _{ 9 }{ x } =\log _{ 9 }{ { y }^{ 2 }zx } =2\Rightarrow { y }^{ 2 }zx={ 3 }^{ 4 }

log 4 z + log 16 x + log 16 y = log 16 z 2 x y = 2 z 2 x y = 4 4 \log _{ 4 }{ z } +\log _{ 16 }{ x } +\log _{ 16 }{ y } =\log _{ 16 }{ { z }^{ 2 }xy } =2\Rightarrow { z }^{ 2 }xy={ 4 }^{ 4 }

We multiply the solutions together to get that ( x y z ) 4 = 24 4 x y z = 24 { \left( xyz \right) }^{ 4 }={ 24 }^{ 4 }\Rightarrow xyz=24

From this it's easy to get the values of x , y , z x,y,z which are equal to 2 3 , 27 8 , 32 3 \frac { 2 }{ 3 } ,\frac { 27 }{ 8 } ,\frac { 32 }{ 3 } respectively.

Therefore the answer is 2 + 3 + 27 + 8 + 32 + 3 = 75 2+3+27+8+32+3=75 .

Sai Ram
Sep 6, 2015

The given equations are

The first equation can be written as

log 2 x + log 4 y + log 4 z = log 4 x 2 y x = 2 x 2 y z = 2 4 . . . . . . . . . . . . 1 \log_{2}{x}+\log_{4}{y}+\log_{4}{z}=\log_{4}{x^2yx}=2 \Rightarrow {x^2}yz=2^4............\boxed{1}

Similarly,

log 3 y + log 9 z + log 9 x = 2 = log 9 y 2 x z y 2 z x = 3 4 . . . . . . . . . . . . 2 \log_{3}{y}+\log_{9}{z}+\log_{9}{x}=2 =\log_{9}{{y^2}xz} \Rightarrow {y^2}zx = 3^4............\boxed{2}

Similarly,

log 4 z + log 16 x + log 16 y = 2 = log 16 z 2 x y z 2 x y = 1 6 2 = 4 4 . . . 3 \log_{4}{z}+\log_{16}{x}+\log_{16}{y}=2=\log_{16}{{z^2}xy} \Rightarrow {z^2}xy=16^2=4^4...\boxed{3}

Multiplying 1 , 2 , 3 1,2,3 ,we get,

( x y z ) 4 = 2 4 × 3 4 × 4 4 = ( 24 ) 4 x y z = 24 (xyz)^{4}=2^4 \times 3^4 \times 4^4 = (24)^4 \Rightarrow \boxed{xyz=24}

Then 1 1 becomes x 2 y z = 16 x ( x y z ) = 16 x = 16 24 x = 2 3 {x^2}yz=16 \Rightarrow x(xyz)=16 \Rightarrow x=\dfrac{16}{24} \Rightarrow \boxed{x=\dfrac{2}{3}}

Similarly 2 2 becomes y 2 x z = 81 y ( x y z ) = 81 y = 81 24 y = 27 8 {y^2}xz=81 \Rightarrow y(xyz)=81 \Rightarrow y=\dfrac{81}{24} \Rightarrow \boxed{y=\dfrac{27}{8}}

Similarly 3 3 becomes z 2 x y = 256 z ( x y z ) = 256 z = 256 24 z = 32 3 {z^2}xy = 256 \Rightarrow z(xyz)=256 \Rightarrow z=\dfrac{256}{24} \Rightarrow \boxed{z=\dfrac{32}{3}}

Therefore the answer is 2 + 3 + 27 + 8 + 32 + 3 = 75. 2+3+27+8+32+3=\boxed{75.}

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