Find α 5 + β 5 + γ 5 , if α , β and γ are the roots of the equation x 3 + 3 x + 3 = 0 .
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Short and sweet :) Same way BTW.
For easy typing purposes, let α = a , β = b and γ = c
x 3 + 3 x + 3 = 0
From Vieta's formula, we know that:
a + b + c = 0 a b + a c + b c = 3 a b c = − 3
Use Newton's sums to find the value of a 5 + b 5 + c 5 :
a 2 + b 2 + c 2 = ( a + b + c ) 2 − 2 ( a b + a c + b c ) = 0 2 − 2 ( 3 ) = − 6 a 3 + b 3 + c 3 = ( a + b + c ) ( a 2 + b 2 + c 2 ) − ( a b + a c + b c ) ( a + b + c ) + 3 a b c = 0 ( − 6 ) − 3 ( 0 ) + 3 ( − 3 ) = − 9 a 4 + b 4 + c 4 = ( a + b + c ) ( a 3 + b 3 + c 3 ) − ( a b + a c + b c ) ( a 2 + b 2 + c 2 ) + a b c ( a + b + c ) = 0 ( − 9 ) − 3 ( − 6 ) + ( − 3 ) ( 0 ) = 1 8 a 5 + b 5 + c 5 = ( a + b + c ) ( a 4 + b 4 + c 4 ) − ( a b + a c + b c ) ( a 3 + b 3 + c 3 ) + a b c ( a 2 + b 2 + c 2 ) = 0 ( 2 7 ) − 3 ( − 9 ) + ( − 3 ) ( − 6 ) = 2 7 + 1 8 = 4 5
⟹ α 5 + β 5 + γ 5 = 4 5
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x 3 + 3 x + 3 = 0
c y c l i c ∑ α = 0 , c y c l i c ∑ α β = 3
x 3 = − 3 x − 3
x 5 = − 3 x 3 − 3 x 2
x 5 = − 3 ( − 3 x − 3 ) − 3 x 2 = 9 x + 9 − 3 x 2
c y c l i c ∑ α 5 = 9 c y c l i c ∑ α + 9 c y c l i c ∑ 1 − 3 c y c l i c ∑ α 2
S = 9 ( 0 ) + 9 ( 3 ) − 3 ( 0 2 − 2 ( 3 ) ) = 2 7 + 1 8 = 4 5