A quartic identity?

Algebra Level 4

x 4 11 x 3 + k x 2 + 269 x 2001 = 0 x^4-11x^3+kx^2+269x-2001=0

The product of two roots of the equation above is 69 -69 , find the value of k k .


The answer is -10.

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1 solution

Daniel Rabelo
Apr 24, 2015

Let the roots be a , b , x , y a,b,x,y and x y = 69 xy=-69

Using Vieta's formulas, we obtain the following system of equations:

x y a b = 2001 xyab=-2001

x y ( a + b ) + a b ( x + y ) = 269 xy(a+b)+ab(x+y)=-269

x y + a b + ( a + b ) ( x + y ) = k xy+ab+(a+b)(x+y)=k

x + y + a + b = 11 x+y+a+b=11

Hence, from the first equation, a b = 29 ab=29 and the system is equivallent to:

29 ( x + y ) 69 ( a + b ) = 269 29(x+y)-69(a+b)=-269

( x + y ) + ( a + b ) = 11 (x+y)+(a+b)=11

k + 40 = ( x + y ) ( a + b ) k+40=(x+y)(a+b)

Writting x + y = u x+y=u and a + b = v a+b=v , we obtain:

29 u 69 v = 269 29u-69v=-269

u + v = 11 u+v=11

Solving this system we obtain: u = 5 , v = 6 u=5, v=6 and k + 40 = u v = 30 k+40=uv=30 . Then, finally:

k = 10 \boxed{k=-10}

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