An Algebra Problem from RMO

Algebra Level 3

{ f ( x ) = x 3 + a x 2 + b x + c g ( x ) = x 3 + b x 2 + c x + a \begin{cases} f(x)={ x }^{ 3 }+{ a }x^{ 2 }+bx+c \\ g(x)={ x }^{ 3 }+{ b }x^{ 2 }+cx+a \end{cases} Given that a , b , c a, b, c are integers with c 0 c\neq 0 and the following conditions hold for the above functions:

(a) f ( 1 ) = 0 f(1) = 0 ;

(b) the roots of g ( x ) = 0 g(x) = 0 are the squares of the roots of f ( x ) = 0 f(x) = 0 .

Find the absolute value of ( a 2013 + b 2013 + c 2013 ) . ({ a }^{ 2013 }+{ b }^{ 2013 }+{ c }^{ 2013 }).


The answer is 1.

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2 solutions

Karthik Sharma
Nov 15, 2014

As f ( 1 ) = 0 f(1)=0 and g ( 1 ) = 0 g(1)=0 , (because f ( 1 ) = g ( 1 ) f(1)=g(1) )

So, 1 is a root of both equations.

Let other roots of f ( x ) = 0 f(x)=0 be m m and n n .

So, the roots of g ( x ) = 0 g(x)=0 will be m 2 m^2 and n 2 n^2 .

Therefore, ( 1 ) ( p ) ( q ) = c (1)(p)(q) = -c and ( 1 ) ( p 2 ) ( q 2 ) = a (1)(p^2)(q^2) = -a

So, a = c 2 -a=c^2

And, a = p + q + 1 -a =p+q+1

Squaring both sides,

a 2 = p 2 + q 2 + 1 + 2 ( p q + p + q ) = b + 2 b = b a^2=p^2 + q^2 + 1 +2(pq+p+q) = -b +2b = b

Therefore, b = c 4 b=c^4 .

And as f ( 1 ) = 0 f(1)=0 , we have,

1 + c c 2 + c 4 = 0 1+c-c^2+c^4 = 0 ( c + 1 ) ( c 3 c 2 + 1 = 0 ) (c+1)(c^3-c^2+1=0)

And, as a , b , c a,b,c are integers, c = 1 \boxed{c=-1}

So, b = 1 \boxed{b=1} and a = 1 \boxed{a=-1}

Therefore, a 2013 + b 2013 + c 2013 = 1 a^{2013}+b^{2013}+c^{2013}= \boxed{-1}

So, a 2013 + b 2013 + c 2013 = 1 |a^{2013}+b^{2013}+c^{2013}|= \boxed{1}

Nice solution.

Ninad Akolekar - 6 years, 6 months ago
Aareyan Manzoor
Dec 7, 2014

Use newtons sum, let α , β , γ \alpha,\beta, \gamma be roots of p ( x ) p(x) let α 2 , β 2 , γ 2 \alpha^2 ,\beta^2, \gamma^2 be roots of g ( x ) g(x) we know α 2 + β 2 + γ 2 = a 2 2 b \alpha^2 +\beta^2+ \gamma^2=a^2 -2b α 2 + β 2 + γ 2 = b \alpha^2 +\beta^2+ \gamma^2=-b we get b = a 2 b=a^2 again α β γ = c \alpha \beta \gamma = -c α 2 β 2 γ 2 = a \alpha^2 \beta^2 \gamma^2 =-a so a = c 2 a=-c^2 since f(1)=0 a + b + c = 1 a+b+c=-1 c 2 + c 4 + c 1 = 0 -c^2 +c^4+c-1=0 and the only integral solution is + 1 +1 so, a = 1 , c = + 1 , b = + 1 a= -1, c=+1,b=+1 solve and get 1 \boxed{1}

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