Sunday, January 22, 2012

Solve the system x+y+2xy=-11 2x^2y+2xy^2=-12

We have to solve


x+y+2xy=-11
...(1)


2x^2y+2xy^2=-12
...(2)


(2)


=> x^2y +
xy^2 = -6


=> xy(x+y) =
-6


(1)


=> x+ y + 2xy =
-11


If we take the variables x+y and xy together as A and
B, we get


A*B = -6


2A + B =
-11


A( -11 - 2A) =
-6


=> 11A + 2A^2 - 6 =
0


=> 2A^2 + 11A - 6 =
0


=> 2A^2 + 12A - A - 6 =
0


=> 2A ( a + 6) - 1(A + 6) =
0


=> (2A - 1)(A + 6) =
0


=> A = 1/2 and A =
-6


=> B = -12 , 1


B =
x+y = -12


A = xy = 1/2


From
this we get the equation


x^2 + 12x - 1/2 =
0


=> 2x^2 + 24x - 1 =
0


x1 = [-24 + sqrt(576 +
8 )]/4


=> x1 = [-24 + sqrt
584]/4


=> x1 = -6 + sqrt 584 /
4


=> y1 = -6 - sqrt 584 /
4


x2 = -6 - sqrt 584 / 4


y2 =
-6 + sqrt 584 / 4


Now, for the values -6 and 1 we
have:


x^2 - x - 6 = 0


x^2 - 3x
+ 2x - 6 =0


=> x(x - 3) + 2( x - 3)
=0


=> (x + 2)(x - 3)
=0


x1 = -2


y1 =
3


x2 = 3


y2 =
-2


So the solutions for x and y are (-2 ,
3),( 3, 2), (-6 + sqrt 584 / 4, -6 - sqrt 584 / 4), ( -6 - sqrt 584 / 4 , -6 + sqrt 584
/ 4)

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