Literally too many variables!-2

Algebra Level 4

If a = b + c x 2 a=\dfrac{b+c}{x-2} , b = c + a y 2 b=\dfrac{c+a}{y-2} , c = a + b z 2 c=\dfrac{a+b}{z-2} , x y + y z + x z = 67 xy+yz+xz=67 and x + y + z = 2010. x+y+z=2010. What is the value of x y z -xyz ?


The answer is 5892.

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1 solution

Chew-Seong Cheong
Nov 30, 2017

Given that a = b + c x 2 x = b + c a + 2 = a + b + c a + 1 = α + 1 a = \dfrac {b+c}{x-2} \implies x = \dfrac {b+c}a + 2 = \dfrac {a+b+c}a + 1 = \alpha + 1 , where α = a + b + c a \alpha =\dfrac {a+b+c}a . Similarly, y = β + 1 y = \beta + 1 and z = γ + 1 z = \gamma +1 . where β = a + b + c b \beta =\dfrac {a+b+c}b and γ = a + b + c c \gamma =\dfrac {a+b+c}c . Then we have:

x + y + z = 2010 α + 1 + β + 1 + γ + 1 = 2010 α + β + γ = 2007 . . . ( 1 ) \begin{aligned} x+y+z & = 2010 \\ \alpha + 1 + \beta + 1 + \gamma + 1 & = 2010 \\ \color{#3D99F6} \alpha + \beta + \gamma & \color{#3D99F6} = 2007 & \color{#3D99F6} ...(1) \end{aligned}

x y + y z + z x = 67 c y c ( α + 1 ) ( β + 1 ) = 67 c y c ( α β + α + β + 1 ) = 67 α β + β γ + γ α + 2 ( α + β + γ ) + 3 = 67 α β + β γ + γ α = 67 2 ( α + β + γ ) 3 ( 1 ) : α + β + γ = 2007 α β + β γ + γ α = 3950 . . . ( 2 ) \begin{aligned} xy+yz+zx & = 67 \\ \sum_{cyc} (\alpha + 1)(\beta + 1) & = 67 \\ \sum_{cyc} (\alpha \beta + \alpha + \beta + 1) & = 67 \\ \alpha \beta + \beta \gamma + \gamma \alpha + 2(\alpha + \beta + \gamma) + 3 & = 67 \\ \alpha \beta + \beta \gamma + \gamma \alpha & = 67 - 2({\color{#3D99F6}\alpha + \beta + \gamma}) - 3 & \small \color{#3D99F6} (1): \quad \alpha + \beta + \gamma = 2007 \\ \color{#D61F06} \alpha \beta + \beta \gamma + \gamma \alpha & \color{#D61F06} = - 3950 & \color{#D61F06} ... (2) \end{aligned}

Now consider

α β + β γ + γ α = ( a + b + c ) 2 ( 1 a b + 1 b c + 1 c a ) = ( a + b + c ) 2 ( a + b + c a b c ) = ( a + b + c ) 3 a b c = α β γ α β γ = 3950 . . . ( 3 ) \begin{aligned} \color{#D61F06} \alpha \beta + \beta \gamma + \gamma \alpha & = (a+b+c)^2 \left(\frac 1{ab} + \frac 1{bc} + \frac 1{ca} \right) \\ & = (a+b+c)^2 \left(\frac {a+b+c}{abc} \right) \\ & = \frac {(a+b+c)^3}{abc} \\ & = \alpha \beta \gamma \\ \implies \color{#D61F06} \alpha \beta \gamma & \color{#D61F06} = - 3950 & \color{#D61F06} ...(3) \end{aligned}

At last,

x y z = ( α + 1 ) ( β + 1 ) ( γ + 1 ) = α β γ + α + β + γ + α β + β γ + γ α + 1 = 3950 + 2007 + 3950 + 1 x y z = 5892 \begin{aligned} xyz & = (\alpha + 1)(\beta + 1)(\gamma + 1) \\ & = {\color{#D61F06}\alpha \beta \gamma} + {\color{#3D99F6}\alpha + \beta + \gamma} + {\color{#D61F06} \alpha \beta + \beta \gamma + \gamma \alpha} + 1 \\ & = {\color{#D61F06}-3950} + {\color{#3D99F6}2007} + {\color{#D61F06}-3950} + 1 \\ \implies - xyz & = \boxed{5892} \end{aligned}

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