Indian Regional Mathematical Olympiad Problem 2

Algebra Level 5

Let f ( x ) = x 3 + a x 2 + b x + c f(x) = x^3 + ax^2 + bx + c and g ( x ) = x 3 + b x 2 + c x + a g(x) = x^3 + bx^2 + cx + a , where a , b , c a, b, c are integers with c 0 c\neq 0 . Suppose that the following conditions hold: (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 value of a 2013 + b 2013 + c 2013 a^{2013}+b^{2013}+c^{2013} .


The answer is -1.

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3 solutions

Parth Lohomi
Mar 21, 2015

So c = ( a + b + 1 ) c = -(a+b+1) . Then f ( x ) = ( x 1 ) ( x 2 + ( a + 1 ) x + ( a + b + 1 ) ) f(x) = (x-1)(x^2 + (a+1)x + (a+b+1)) and g ( x ) = ( x 1 ) ( x 2 + ( b + 1 ) x a ) g(x) = (x-1)(x^2 + (b+1)x - a) Let 1 , u , v 1,u,v be the roots of f f . It follows ( b + 1 ) = u 2 + v 2 = ( u + v ) 2 2 u v = ( a + 1 ) 2 2 ( a + b + 1 ) -(b+1) = u^2+v^2 = (u+v)^2 - 2uv = (a+1)^2 - 2(a+b+1) , whence b = a 2 b = a^2 , and a = u 2 v 2 = ( a + b + 1 ) 2 -a = u^2v^2 = (a+b+1)^2 , whence a = c 2 a = -c^2 . It follows 0 = 1 + a + b + c = 1 c 2 + c 4 + c = ( c + 1 ) ( c 3 c 2 + 1 ) 0 = 1+a+b+c = 1-c^2 +c^4 + c = (c+1)(c^3-c^2+1) , whence c = 1 c=-1 , and so a = 1 a=-1 and b = 1 b=1 . Therefore a 2013 + b 2013 + c 2013 = 1 \boxed{a^{2013} + b^{2013} + c^{2013} = -1}

Very neat solution, upvoted !

Venkata Karthik Bandaru - 6 years, 2 months ago

@Parth Lohomi you have also posted this question before.

Shubhendra Singh - 6 years, 2 months ago

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Ya I knew that @shubhendra singh .Thanks ¨ \ddot\smile

Parth Lohomi - 6 years, 2 months ago

Good one,upvoted!

rajdeep brahma - 4 years, 2 months ago
Aakash Khandelwal
May 20, 2015

by relationship between a,b and c we get

a+b+c=-1

b=a^2

-a=c^2

therefore c^4-c^2+c+1=0

its 1 root is -1

therefore a=c=-1 and b=1

hence answer=-1+1-1=-1

Alok Sharma
Mar 28, 2015

considering the roots of f(x)=0, as p,q and r- we see the roots of g(x)=0 as 1/p, 1/q and 1/r. Also, i) p+q+r=-1 ii) 1/p, 1/q and 1/r is same as p^2, q^2 and r^2. iii) fact that pqr is not 0. two of them are -1 and remaining is 1.

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