If x 2 + x + 1 = 0 , what is the value of x 6 + x 6 1 ?
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Using the quadratic formula (with a=b=c=1) gives x 1 = - 2 1 + 2 3 j and x 2 = - 2 1 - 2 3 j , where j = − 1 .
Using De Moivre's: x 1 6 = [- 2 1 + 2 3 j ]^6 = [Cos 3 2 π + j Sin 3 2 π ]^6 = Cos4 π + j Sin 4 π = 1.
Similarily: x 2 6 = [- 2 1 - 2 3 j ]^6 = [Cos 3 4 π + j Sin 3 4 π ]^6 = Cos8 π + j Sin 8 π = 1.
∴ x 6 + x 6 1 = 2
Beautiful solution. Nice use of De Moivre's !
Since x 2 + x 2 1 = − 1 .Cube it ( x 2 + x 2 1 = − 1 ) 3 Produces x 6 + 3 ( x 2 + x 2 1 ) + x 6 1 = − 1 Substitute x 6 + 3 ( − 1 ) + x 6 1 = − 1 x 6 + x 6 1 = 2
Solution to the first equation is ( − 1 ) 2 / 3
Simply, putting the value in second equation, we get 1 + 1 = 2
solution of equation 1 are omega and omega square put the value in the question asked so we get 1+1=2 answer
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x is a cube root of unity:
x 2 + x + 1 = x − 1 x 3 − 1 = 0 ⟹ x 3 − 1 = 0 ⟹ x 3 = 1
So x 6 + x 6 1 = ( x 3 ) 2 + ( x 3 ) 2 1 = 1 2 + 1 2 1 = 1 + 1 = 2