Let and be real numbers such that
If , find the value of .
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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 )
From ( 2 ) we get
x y ( x + y ) = 2 ⟺ x + y = x y 2
Manipulating ( 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
Substituting what we get from ( 2 )
( x y 2 ) 2 = 7 − 3 x y ⟺ x 2 y 2 4 = 7 − 3 x y ⟺ 3 x 3 y 3 − 7 x 2 y 2 + 4 = 0
A cubic in x y which can be factored as
( x y − 1 ) ( x y − 2 ) ( x y + 3 2 ) = 0
Which implies x y ∈ { 1 , 2 , − 3 2 }
x y = 1 can be immediately discarded, since
x + y = x y 2 = 2
But x + y = 2 from the problem hypotesis
x y = 2 implies x + y = 1 , so x and y are the solutions of the equation
t 2 − 1 t + 2 = 0
Which has no real solutions. x y = − 3 2 implies x + y = − 3 , so x and y are the solutions of
t 2 − ( − 3 ) t + − 3 2 = 0 ⟺ 3 t 2 + 9 t − 2 = 0
Which do have real solutions, thus x y = − 3 2 . To conclude
( 6 x y ) 2 = [ 6 ( − 3 2 ) ] 2 = ( − 4 ) 2 = 1 6