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Algebra Level 4

Let f ( x ) = x 3 + a x 3 + b x + c , g ( x ) = x 3 + b x 2 + c x + a f(x) = x^3+ax^3+bx+c,g(x)=x^3+bx^2+cx+a for integers a , b , c a,b,c with c 0 c\ne 0 . Suppose the following conditions hold:

  1. f ( 1 ) = 0 f(1)=0 ,

  2. The roots of g ( x ) = 0 g(x)=0 are square 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} .

Source: RMO 2013.


The answer is -1.

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1 solution

Aritra Jana
Nov 7, 2014

We have:

f ( x ) = x 3 + a x 2 + b x + c f(x)=x^{3}+ax^{2}+bx+c ; g ( x ) = x 3 + b x 2 + c x + a g(x)=x^{3}+bx^{2}+cx+a where a , b , c I a,b,c\in\mathbb{I} and c 0 c\neq0

Also, since 0 = f ( 1 ) = 1 + a + b + c = g ( 1 ) 0=f(1)=1+a+b+c=g(1) we have 1 1 as the common root of both the polynomials.

Now, Let's write down the expressions we get by Vieta's Formulae considering α \alpha and β \beta as the other two roots for f ( x ) f(x) and α 2 \alpha^{2} and β 2 \beta^{2} as the other roots for g ( x ) g(x)

1 + α + β = a 1+\alpha+\beta=-a ........... ( i ) (i)

α β + α + β = b \alpha\beta+\alpha+\beta=b .................. ( i i ) (ii)

α β = c \alpha\beta=-c ................. ( i i i ) (iii)

1 + α 2 + β 2 = b 1+\alpha^{2}+\beta^{2}=-b ............... ( i v ) (iv)

α 2 β 2 = a \alpha^{2}\beta^{2}=-a ........... ( v ) (v)

These are all we need.

Notice, from ( i i i ) (iii) and ( v ) (v) : c 2 = a c^{2}=-a

also, ( i v ) (iv) can be written as: ( 1 + α + β ) 2 2 ( α + β + α β ) = b (1+\alpha+\beta)^{2}-2(\alpha+\beta+\alpha\beta)=-b ............... ( v i ) (vi)

and 2 ( α + β + α β ) = 2 b 2(\alpha+\beta+\alpha\beta)=2b ..........from ( i i ) (ii)

Thus, by putting respective values in ( v i ) (vi) we get:

a 2 = b a^{2}=b

So: a 2 = b = c a^{2}=b=-c

We also have 1 + a + b + c = 0 1+a+b+c=0 .

So: a = 1 a=-1

Our required expression: a 2013 + b 2013 + c 2013 = a 2013 = 1 \large{a^{2013}+b^{2013}+c^{2013}=a^{2013}=\boxed{-1}}


Maybe :)

You are a person worth following :)

Parth Lohomi - 6 years, 7 months ago

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well, thanks. But you are not even my age, and still have more intellect than me.

P.S.: You post some great problems man!! :D

Aritra Jana - 6 years, 7 months ago

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Oh thanks I will keep on posting more problems

Parth Lohomi - 6 years, 7 months ago

This exact problem appeared in RMO, 2013. @Parth Lohomi , maybe you'd like to mention at the end of the problem.

Satvik Golechha - 6 years, 7 months ago

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Mentioned it

Parth Lohomi - 6 years, 7 months ago

Exactly ! I did the same . :)

Keshav Tiwari - 6 years, 6 months ago

i know it is unfair to do this but here is my method ....

Since a and b are integers , consider a = b = 0 a=b=0

c = 1 Answer: 1 \implies c=-1 \\ \text{Answer: } -1

:P

Sabhrant Sachan - 4 years, 9 months ago

Did the same!

Kartik Sharma - 6 years, 7 months ago

Perfect Vieta Application !

Venkata Karthik Bandaru - 6 years, 1 month ago

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