Saturday, October 24, 2015

Calculate (z1+1)^10+(z2+1)^10. z1, z2 are the solutions of equation z^2+z+1.

If z^2 + z + 1 = 0, that means that if we'll multiply both
sides by (z-1), we'll get:


(z-1)(z^2 + z + 1) =
0


But the product is the result of difference of
cubes:


z^3 - 1 = (z-1)(z^2 + z +
1)


If (z-1)(z^2 + z + 1) = 0, then z^3 - 1 =
0


We'll add 1 both sides:


z^3
= 1


z1 and z2 are the roots of the equation z^3 - 1 = 0,
so:


z1^3 = 1


z2^3  =
1


From the equation z^2 + z + 1 =
0


z + 1 = -z^2


We'll
substitute z1 and z2:


z1 + 1 =
-z1^2


z2 + 1 = -z2^2


(z1 +
1)^10 + (z2 + 1)^10 = (-z1^2)^10 + (-z2^2)^10


We'll
re-write the expression (-z1^2)^10 + (-z2^2)^10:


(-z1^2)^10
+ (-z2^2)^10 = (z1^2)^10 + (z2^2)^10


(z1^10)^2 +
(z2^10)^2


We'll write z1^10 =
z1^(9+1)


z1^(9+1)
= z1^9*z1


z1^9*z1 =
(z1^3)^2*z1


But z1^3 = 1


z1^10
= 1^2*z1


z1^10 = z1


z2^10 =
z2


So, 


(z1^10)^2 + (z2^10)^2
= z1^2 + z2^2


We'll use Viete's relations to express z1 +
z2:


z1 + z2 = -b/a


where b and
a are the coefficients of the quadratic equation


az^2 + bz
+ c = 0


In our case, the quadratic
is:


z^2 +z +1 = 0


a =
1


b = 1


c =
1


z1 + z2 = -1/1


z1 + z2 =
-1


z1*z2 = c/a


z1*z2 =
1


z1^2 + z2^2 = (z1 + z2)^2 -
2z1*z2


(z1 + 1)^10 + (z2 + 1)^10 = (-1)^2 -
2*1


(z1 + 1)^10 + (z2 + 1)^10 = 1 -
2


(z1 + 1)^10 + (z2 + 1)^10 =
-1

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