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Algebra Level 5

2000 x 6 + 100 x 5 + 10 x 3 + x 2 = 0 2000x^6+100x^5+10x^3+x-2=0

The equation above has two real roots one of which is m + n r \dfrac{m+\sqrt{n}}{r} for integers m , n , r m,n,r with n n square free and r r positive.

Find m + n + r m+n+r .


The answer is 200.

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1 solution

Parth Lohomi
Apr 20, 2015

We may factor the equation as:

2000 x 6 + 100 x 5 + 10 x 3 + x 2 = 0 2 ( 1000 x 6 1 ) + x ( 100 x 4 + 10 x 2 + 1 ) = 0 2 [ ( 10 x 2 ) 3 1 ] + x [ ( 10 x 2 ) 2 + ( 10 x 2 ) + 1 ] = 0 2 ( 10 x 2 1 ) [ ( 10 x 2 ) 2 + ( 10 x 2 ) + 1 ] + x [ ( 10 x 2 ) 2 + ( 10 x 2 ) + 1 ] = 0 ( 20 x 2 + x 2 ) ( 100 x 4 + 10 x 2 + 1 ) = 0 \begin{aligned} 2000x^6+100x^5+10x^3+x-2&=0\\ 2(1000x^6-1) + x(100x^4+10x^2+1)&=0\\ 2[(10x^2)^3-1]+x[(10x^2)^2+(10x^2)+1]&=0\\ 2(10x^2-1)[(10x^2)^2+(10x^2)+1]+x[(10x^2)^2+(10x^2)+1]&=0\\ (20x^2+x-2)(100x^4+10x^2+1)&=0\\ \end{aligned}

Now 100 x 4 + 10 x 2 + 1 1 > 0 100x^4+10x^2+1\ge 1>0 for real x x . Thus the real roots must be the roots of the equation 20 x 2 + x 2 = 0 20x^2+x-2=0 . By the quadratic formula the roots of this are:

x = 1 ± 1 2 4 ( 2 ) ( 20 ) 40 = 1 ± 1 + 160 40 = 1 ± 161 40 . x=\dfrac{-1\pm\sqrt{1^2-4(-2)(20)}}{40} = \dfrac{-1\pm\sqrt{1+160}}{40} = \frac{-1\pm\sqrt{161}}{40}. Thus r = 1 + 161 40 r=\dfrac{-1+\sqrt{161}}{40} , and so the final answer is 1 + 161 + 40 = 200 -1+161+40 = \boxed{200} .

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