Non-linear System of Equations

Algebra Level 3

x + log ( x ) = y 1 y + log ( y 1 ) = z 1 z + log ( z 2 ) = x + 2 \begin{aligned} x+\log{(x)} = y-1\\ y+\log{(y-1)} = z-1 \\ z+\log{(z-2)} = x+2 \\ \end{aligned}

There is only one solution set ( x , y , z ) \left(x,y,z\right) to the system of equations above. Evaluate x + y + z x+y+z .

Note: Take the base of the logarithms as 10.

Image Credit: Wikimedia Thoma Loneliness .


The answer is 6.

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3 solutions

Dylan Pentland
Jun 30, 2015

Let x = x , y = y 1 , z = z 2 x'=x, y'=y-1, z'=z-2 . Then:

x + log ( x ) = y x'+\log(x')=y'

Substituting,

y + log ( y ) = z y'+\log(y')=z'

then again to get

z + log ( z ) = x z'+\log(z')=x'

and more substitution, this time substitute for the first y y' in the second equation then use that for first z z' in the last equation:

x + log ( x ) + log ( y ) + log ( z ) = x x'+\log(x')+\log(y')+\log(z')=x'

Now it's obvious that if log ( x ) = log ( y ) = log ( z ) = 0 \log(x')=\log(y')=\log(z')=0 then we have a solution hence x = 1 , y = 1 , z = 1 x'=1, y'=1, z'=1 and x = 1 , y = 2 , z = 3 x=1, y=2, z=3 . We know that log ( x ) , log ( y ) , log ( z ) 0 \log(x'), \log(y'), \log(z')\ge0 : observe that if log ( z ) < 0 \log(z')<0 then z + log ( z ) = x z'+\log(z')=x' hence z > x z'>x' and log ( x ) \log(x') is also less than 0. This method can be used to produce a sort of 'chain reaction' meaning if any one of them is negative we can use this repeatedly to show that all are negative. So the solution found is unique.

Great solution. This is how its done, with a uniqueness proof.

Steven Zheng - 5 years, 11 months ago

How did you get to log(x') =log(y') =log(z')?

Katie Marsden - 5 years, 11 months ago

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From the equation above that I conclude that l o g ( x ) + l o g ( y ) + l o g ( z ) = 0 log(x')+log(y')+log(z')=0 and notice that if you set all the logs to 0 this works. After that I prove that each log by itself is strictly greater than zero, so the sum is only zero when you set each to zero.

Dylan Pentland - 5 years, 11 months ago

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Oh yeah I see-great solution!

Katie Marsden - 5 years, 11 months ago
Chew-Seong Cheong
Jun 30, 2015

It is given that:

{ x + log ( x ) = y 1 . . . ( 1 ) y + log ( y 1 ) = z 1 . . . ( 2 ) z + log ( z 2 ) = x + 2 . . . ( 3 ) \begin{cases} x + \log{(x)} & = y-1 &...(1) \\ y + \log{(y-1)} & = z-1 &...(2) \\ z + \log{(z-2)} & = x+2 &...(3) \end{cases}

( 1 ) + ( 2 ) + ( 3 ) : (1)+(2)+(3):

x + y + z + log ( x ) + log ( y 1 ) + log ( z 2 ) = y + z + x 1 1 + 2 log ( x ) + log ( y 1 ) + log ( z 2 ) = 0 x ( y 1 ) ( z 2 ) = 1 \begin{aligned} x + y + z + \log{(x)} + \log{(y-1)} + \log{(z-2)} & = y+z+x -1-1+2 \\ \Rightarrow \log{(x)} + \log{(y-1)} + \log{(z-2)} & = 0 \\ \Rightarrow x(y-1)(z-2) & = 1 \end{aligned}

We note that x 1 x \ne -1 , y 1 1 y - 1 \ne -1 and z 2 1 z-2 \ne -1 because log ( 1 ) \log{(-1)} is undefined, there is only one trivial solution:

{ x = 1 x = 1 y 1 = 1 y = 2 z 2 = 1 z = 3 \begin{cases} x = 1 & \Rightarrow x = 1 \\ y -1 = 1 & \Rightarrow y = 2 \\ z -2 = 1 & \Rightarrow z = 3 \end{cases}

Since there is only one solution, therefore the trivial solution is the unique solution and x + y + z = 1 + 2 + 3 = 6 x+y+z = 1+2+3 = \boxed{6}

Moderator note:

Your solution is incomplete. You need to explain why solutions like x = 1 , y 1 = 1 , z 2 = 1 x = 1, y - 1 = -1, z - 2 = -1 is not allowed.

Logarithmic function is defined for positive arguments only so for y-1=-1 log is not defined

Mayank Saggar - 5 years, 11 months ago

Thanks, I have changed the solution accordingly.

Chew-Seong Cheong - 5 years, 11 months ago
Gulbagh Singh
Jul 15, 2015

Adding the three equations we get Log x + log (y-1)+ log(z-2) =0; Log(x)(y-1)(z-2)=0; X(y-1)(z-2)=1; Which has multiple solutions.. Where is this wrong? Please excuse the lack of formatting.

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