x + lo g ( x ) = y − 1 y + lo g ( y − 1 ) = z − 1 z + lo g ( z − 2 ) = x + 2
There is only one solution set ( x , y , z ) to the system of equations above. Evaluate x + y + z .
Note: Take the base of the logarithms as 10.
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Great solution. This is how its done, with a uniqueness proof.
How did you get to log(x') =log(y') =log(z')?
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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 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.
It is given that:
⎩ ⎪ ⎨ ⎪ ⎧ x + lo g ( x ) y + lo g ( y − 1 ) z + lo g ( z − 2 ) = y − 1 = z − 1 = x + 2 . . . ( 1 ) . . . ( 2 ) . . . ( 3 )
( 1 ) + ( 2 ) + ( 3 ) :
x + y + z + lo g ( x ) + lo g ( y − 1 ) + lo g ( z − 2 ) ⇒ lo g ( x ) + lo g ( y − 1 ) + lo g ( z − 2 ) ⇒ x ( y − 1 ) ( z − 2 ) = y + z + x − 1 − 1 + 2 = 0 = 1
We note that x = − 1 , y − 1 = − 1 and z − 2 = − 1 because lo g ( − 1 ) is undefined, there is only one trivial solution:
⎩ ⎪ ⎨ ⎪ ⎧ x = 1 y − 1 = 1 z − 2 = 1 ⇒ x = 1 ⇒ y = 2 ⇒ z = 3
Since there is only one solution, therefore the trivial solution is the unique solution and x + y + z = 1 + 2 + 3 = 6
Your solution is incomplete. You need to explain why solutions like 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
Thanks, I have changed the solution accordingly.
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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Let x ′ = x , y ′ = y − 1 , z ′ = z − 2 . Then:
x ′ + lo g ( x ′ ) = y ′
Substituting,
y ′ + lo g ( y ′ ) = z ′
then again to get
z ′ + lo g ( z ′ ) = x ′
and more substitution, this time substitute for the first y ′ in the second equation then use that for first z ′ in the last equation:
x ′ + lo g ( x ′ ) + lo g ( y ′ ) + lo g ( z ′ ) = x ′
Now it's obvious that if lo g ( x ′ ) = lo g ( y ′ ) = lo g ( z ′ ) = 0 then we have a solution hence x ′ = 1 , y ′ = 1 , z ′ = 1 and x = 1 , y = 2 , z = 3 . We know that lo g ( x ′ ) , lo g ( y ′ ) , lo g ( z ′ ) ≥ 0 : observe that if lo g ( z ′ ) < 0 then z ′ + lo g ( z ′ ) = x ′ hence z ′ > x ′ and lo g ( 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.