{ f ( x ) = x 3 + a x 2 + b x + c g ( x ) = x 3 + b x 2 + c x + a Given that a , b , c are integers with c = 0 and the following conditions hold for the above functions:
(a) f ( 1 ) = 0 ;
(b) the roots of g ( x ) = 0 are the squares of the roots of f ( x ) = 0 .
Find the absolute value of ( a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 ) .
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Nice solution.
Use newtons sum, let α , β , γ be roots of p ( x ) let α 2 , β 2 , γ 2 be roots of g ( x ) we know α 2 + β 2 + γ 2 = a 2 − 2 b α 2 + β 2 + γ 2 = − b we get b = a 2 again α β γ = − c α 2 β 2 γ 2 = − a so a = − c 2 since f(1)=0 a + b + c = − 1 − c 2 + c 4 + c − 1 = 0 and the only integral solution is + 1 so, a = − 1 , c = + 1 , b = + 1 solve and get 1
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As f ( 1 ) = 0 and g ( 1 ) = 0 , (because f ( 1 ) = g ( 1 ) )
So, 1 is a root of both equations.
Let other roots of f ( x ) = 0 be m and n .
So, the roots of g ( x ) = 0 will be m 2 and n 2 .
Therefore, ( 1 ) ( p ) ( q ) = − c and ( 1 ) ( p 2 ) ( q 2 ) = − a
So, − a = c 2
And, − a = p + q + 1
Squaring both sides,
a 2 = p 2 + q 2 + 1 + 2 ( p q + p + q ) = − b + 2 b = b
Therefore, b = c 4 .
And as f ( 1 ) = 0 , we have,
1 + c − c 2 + c 4 = 0 ( c + 1 ) ( c 3 − c 2 + 1 = 0 )
And, as a , b , c are integers, c = − 1
So, b = 1 and a = − 1
Therefore, a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 = − 1
So, ∣ a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 ∣ = 1