SOLUTION: x=3+root8, then (x^6+x^4+x^2+1)/x^3 ans:204 shortcut formula as possible we have to solve this in 2 minutes

Algebra ->  Square-cubic-other-roots -> SOLUTION: x=3+root8, then (x^6+x^4+x^2+1)/x^3 ans:204 shortcut formula as possible we have to solve this in 2 minutes       Log On


   



Question 1042993: x=3+root8, then (x^6+x^4+x^2+1)/x^3
ans:204
shortcut formula as possible
we have to solve this in 2 minutes

Answer by ikleyn(52835) About Me  (Show Source):
You can put this solution on YOUR website!
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x=3+root8, then (x^6+x^4+x^2+1)/x^3
ans:204
shortcut formula as possible
we have to solve this in 2 minutes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1.  Let us consider these two numbers: x = 3+%2B+sqrt%288%29 and y = 3-sqrt%288%29.

    It is easy to check that x*y = %283%2Bsqrt%288%29%29%2A%283-sqrt%288%29%29 = 3%5E2+-+%28sqrt%288%29%29%5E2 = 9 - 8 = 1.    (1)

    Thus  y = x%5E%28-1%29;  x  and  y are reciprocal.

    Also notice that x+%2B+1%2Fx = x+%2B+y = %283+%2B+sqrt%288%29%29+%2B+%283-sqrt%288%29%29 = 6.                      (2)

    Also notice that %28x+%2B+1%2Fx%29%5E2 = x%5E2+%2B+2+%2B+1%2Fx%5E2 = 6%5E2 = 36,

         therefore x%5E2+%2B+1%2Fx%5E2 = 36 - 2 = 34.                                      (3)

    Armed with this knowldge, we can make the next step.


2.  Let us regroup

      %28x%5E6%2Bx%5E4%2Bx%5E2%2B1%29%2Fx%5E3 = %28x%5E6+%2B+x%5E4%29%2Fx%5E3 + %28x%5E2+%2B+1%29%2Fx%5E3 = %28%28x%5E6+%2B+x%5E4%29%2Fx%5E5%29%2Ax%5E2 + %28%28%28x%5E2+%2B+1%29%2Fx%29%29%2Fx%5E2 = 

    = %28x%2By%29%2Ax%5E2 + %28x%2By%29%2Fx%5E2 =     ( now substitute x + y = 6, according to (2) )

    = 6x%5E2 + 6%2Fx%5E2 = 6%2A%28x%5E2+%2B+1%2Fx%5E2%29 = 6*34 = 204.   ( I substituted  x%5E2 + 1%2Fx%5E2 = 34, according to (3) )

Answer.  %28x%5E6%2Bx%5E4%2Bx%5E2%2B1%29%2Fx%5E3 = 204.

Solved.


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