It's not 1 and 1

Algebra Level 3

Let x x and y y be real numbers such that

{ x 2 + 5 x y + y 2 = 7 x 2 y + x y 2 = 2 \large \begin{cases} x^2+5xy+y^2=7 \\ x^2y+xy^2=2 \end{cases}

If x + y 2 x+y\neq2 , find the value of ( 6 x y ) 2 (6xy)^2 .


The answer is 16.

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1 solution

Marco Brezzi
Aug 13, 2017

We have the following system of equations

{ x 2 + 5 x y + y 2 = 7 ( 1 ) x 2 y + x y 2 = 2 c c c i ( 2 ) \begin{cases} x^2+5xy+y^2=7 \qquad\qquad (1)\\ x^2y+xy^2=2 \qquad\qquad \phantom{ccci} (2) \end{cases}

From ( 2 ) (2) we get

x y ( x + y ) = 2 x + y = 2 x y xy(x+y)=2 \iff x+y=\dfrac{2}{xy}

Manipulating ( 1 ) (1) :

x 2 + 5 x y + y 2 = 7 x 2 + 2 x y + y 2 = 7 3 x y ( x + y ) 2 = 7 3 x y \begin{aligned} x^2+5xy+y^2=7 &\iff x^2+2xy+y^2=7-3xy\\ &\iff (x+y)^2=7-3xy \end{aligned}

Substituting what we get from ( 2 ) (2)

( 2 x y ) 2 = 7 3 x y 4 x 2 y 2 = 7 3 x y 3 x 3 y 3 7 x 2 y 2 + 4 = 0 \begin{aligned} \left(\dfrac{2}{xy}\right)^2=7-3xy &\iff \dfrac{4}{x^2y^2}=7-3xy\\ &\iff 3x^3y^3-7x^2y^2+4=0 \end{aligned}

A cubic in x y xy which can be factored as

( x y 1 ) ( x y 2 ) ( x y + 2 3 ) = 0 (xy-1)(xy-2)\left(xy+\dfrac{2}{3}\right)=0

Which implies x y { 1 , 2 , 2 3 } xy\in\left\{1,2,-\dfrac{2}{3}\right\}

x y = 1 xy=1 can be immediately discarded, since

x + y = 2 x y = 2 x+y=\dfrac{2}{xy}=2

But x + y 2 x+y\neq 2 from the problem hypotesis

x y = 2 xy=\mathbin{\color{#D61F06}2} implies x + y = 1 x+y=\mathbin{\color{#3D99F6}1} , so x x and y y are the solutions of the equation

t 2 1 t + 2 = 0 t^2-\mathbin{\color{#3D99F6}1}t+\mathbin{\color{#D61F06}2}=0

Which has no real solutions. x y = 2 3 xy=\mathbin{\color{#D61F06}-\dfrac{2}{3}} implies x + y = 3 x+y=\mathbin{\color{#3D99F6}-3} , so x x and y y are the solutions of

t 2 ( 3 ) t + 2 3 = 0 3 t 2 + 9 t 2 = 0 t^2-\mathbin{\color{#3D99F6}(-3)}t+\mathbin{\color{#D61F06}-\dfrac{2}{3}}=0\iff 3t^2+9t-2=0

Which do have real solutions, thus x y = 2 3 xy=-\dfrac{2}{3} . To conclude

( 6 x y ) 2 = [ 6 ( 2 3 ) ] 2 = ( 4 ) 2 = 16 (6xy)^2=\left[6\left(-\dfrac{2}{3}\right)\right]^2=(-4)^2=\boxed{16}

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