The product of two roots of the equation above is , find the value of .
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Let the roots be a , b , x , y and x y = − 6 9
Using Vieta's formulas, we obtain the following system of equations:
x y a b = − 2 0 0 1
x y ( a + b ) + a b ( x + y ) = − 2 6 9
x y + a b + ( a + b ) ( x + y ) = k
x + y + a + b = 1 1
Hence, from the first equation, a b = 2 9 and the system is equivallent to:
2 9 ( x + y ) − 6 9 ( a + b ) = − 2 6 9
( x + y ) + ( a + b ) = 1 1
k + 4 0 = ( x + y ) ( a + b )
Writting x + y = u and a + b = v , we obtain:
2 9 u − 6 9 v = − 2 6 9
u + v = 1 1
Solving this system we obtain: u = 5 , v = 6 and k + 4 0 = u v = 3 0 . Then, finally:
k = − 1 0