Hexotic Equation - 1

Algebra Level 4

Find the sum of all integral roots of the equation 6 x 6 25 x 5 + 31 x 4 31 x 2 + 25 x 6 = 0 6x^6-25x^5+31x^4-31x^2+25x-6=0


The answer is 2.

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3 solutions

6 x 6 25 x 5 + 31 x 4 31 x 2 + 25 x 6 = 0 6 ( x 6 1 ) 25 x ( x 4 1 ) + 31 x 2 ( x 2 1 ) = 0 6 ( x 2 1 ) ( x 4 + x 2 + 1 ) 25 x ( x 2 1 ) ( x 2 + 1 ) + 31 x 2 ( x 2 1 ) = 0 ( x 2 1 ) ( 6 x 4 25 x 3 + 37 x 2 25 x + 6 ) = 0 ( x 2 1 ) ( 6 x 4 12 x 3 13 x 3 + 26 x 2 + 11 x 2 22 x 3 x + 6 ) = 0 ( x 2 1 ) ( 6 x 3 ( x 2 ) 13 x 2 ( x 2 ) + 11 x ( x 2 ) 3 ( x 2 ) ) = 0 ( x 2 1 ) ( x 2 ) ( 6 x 3 13 x 2 + 11 x 3 ) = 0 ( x 2 1 ) ( x 2 ) ( 6 x 3 3 x 2 10 x 2 + 5 x + 6 x 3 ) = 0 ( x 2 1 ) ( x 2 ) ( 3 x 2 ( 2 x 1 ) 5 x ( 2 x 1 ) + 3 ( 2 x 1 ) ) = 0 ( x 2 1 ) ( x 2 ) ( 2 x 1 ) ( 3 x 2 5 x + 3 ) = 0 \begin{aligned} 6x^6 - 25x^5 + 31x^4 - 31x^2 + 25x - 6 & = 0 \\ 6(x^6 - 1) - 25x(x^4-1) + 31x^2(x^2-1) & = 0 \\ 6(x^2-1)(x^4+x^2+1) - 25x(x^2-1)(x^2+1) +31x^2(x^2-1) & = 0 \\ (x^2-1)(6x^4-25x^3+37x^2-25x+6) & = 0 \\ (x^2-1)(6x^4-12x^3-13x^3+26x^2+11x^2-22x-3x+6) & = 0 \\ (x^2-1)(6x^3(x-2)-13x^2(x-2)+11x(x-2)-3(x-2)) & = 0 \\ (x^2-1)(x-2)(6x^3-13x^2+11x-3) & = 0 \\ (x^2-1)(x-2)(6x^3-3x^2-10x^2+5x+6x-3) & = 0 \\ (x^2-1)(x-2)(3x^2(2x-1)-5x(2x-1)+3(2x-1)) & = 0 \\ (x^2-1)(x-2)(2x-1)(3x^2-5x + 3) & = 0 \end{aligned}

We note that 3 x 2 5 x + 3 = 0 3x^2-5x + 3 = 0 has no real root, therefore the sum of integral real roots is 1 1 + 2 = 2 1-1+2 = \boxed{2}

Arturo Presa
Sep 16, 2015

Obviously x = 0 x=0 is not a solution of the given equation. So dividing both sides of the equation by x 3 x^{3} we obtain the equivalent equation 6 x 3 25 x 2 + 31 x 31 x + 25 x 2 6 x 3 = 0. 6x^3-25x^2+31x-\frac{31}{x}+\frac{25}{x^2}-\frac{6}{x^{3}}=0. Rearranging terms we get 6 ( x 3 1 x 3 ) 25 ( x 2 1 x 2 ) + 31 ( x 1 x ) = 0. 6(x^3-\frac{1}{x^{3}})-25(x^2-\frac{1}{x^2})+31(x-\frac{1}{x})=0. Factoring x 1 x x-\frac{1}{x} out we get ( x 1 x ) ( 6 ( x 2 + 1 x 2 + 1 ) 25 ( x + 1 x ) + 31 ) = 0. (x-\frac{1}{x})(6(x^2+\frac{1}{x^{2}}+1)-25(x+\frac{1}{x})+31)=0. The latter equation can be rewritten as ( x 1 x ) ( 6 ( x + 1 x ) 2 25 ( x + 1 x ) + 25 ) = 0. (x-\frac{1}{x})(6(x+\frac{1}{x})^{2}-25(x+\frac{1}{x})+25)=0. Therefore we get that x 1 x = 0 , x-\frac{1}{x}=0, x + 1 x = 5 3 , x+\frac{1}{x}=\frac{5}{3}, and x + 1 x = 5 2 . x+\frac{1}{x}=\frac{5}{2}. Solving these three equations we get that the solutions are 1 , 1 , 2 , 1 2 , 5 + i 11 6 , 1, -1, 2, \frac{1}{2}, \frac{5+i\sqrt{11}}{6}, and 5 i 11 6 . \frac{5-i\sqrt{11}}{6}. Therefore the sum of the integer roots is 1 + ( 1 ) + 2 = 2. 1+(-1)+2=2.

Used the same method.

Niranjan Khanderia - 4 years, 11 months ago

nice method

王 征宇 - 3 years, 10 months ago
Rab Gani
Jul 12, 2017

Using trial and error (use the factors of 6) , we get x=1,or -1,
6x^6 – 25x^5 + 31x^4 – 31x^2 + 25x – 6 = (x^2 – 1)(6x^4 - 25x^3 + 37x^2 - 25x + 6) =0 Repeat again, then x=2, So (6x^4 - 25x^3 + 37x^2 - 25x + 6) = (x – 2)(6x^3 - 13x^2 + 11x – 3) = 0 Also x=1/2,is the solution of (6x^3 - 13x^2 + 11x – 3) = (2x – 1)(3x^2 - 5x +3) = 0.So the sum of integral roots = 1 - 1 + 2 = 2

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