Let f ( x ) = x 3 + a x 3 + b x + c , g ( x ) = x 3 + b x 2 + c x + a for integers a , b , c with c = 0 . Suppose the following conditions hold:
f ( 1 ) = 0 ,
The roots of g ( x ) = 0 are square of the roots of f ( x ) = 0 .
Find the value of a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 .
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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
This exact problem appeared in RMO, 2013. @Parth Lohomi , maybe you'd like to mention at the end of the problem.
Exactly ! I did the same . :)
i know it is unfair to do this but here is my method ....
Since a and b are integers , consider a = b = 0
⟹ c = − 1 Answer: − 1
:P
Did the same!
Perfect Vieta Application !
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We have:
f ( x ) = x 3 + a x 2 + b x + c ; g ( x ) = x 3 + b x 2 + c x + a where a , b , c ∈ I and c = 0
Also, since 0 = f ( 1 ) = 1 + a + b + c = g ( 1 ) we have 1 as the common root of both the polynomials.
Now, Let's write down the expressions we get by Vieta's Formulae considering α and β as the other two roots for f ( x ) and α 2 and β 2 as the other roots for g ( x )
These are all we need.
Notice, from ( i i i ) and ( v ) : c 2 = − a
also, ( i v ) can be written as: ( 1 + α + β ) 2 − 2 ( α + β + α β ) = − b ............... ( v i )
and 2 ( α + β + α β ) = 2 b ..........from ( i i )
Thus, by putting respective values in ( v i ) we get:
a 2 = b
So: a 2 = b = − c
We also have 1 + a + b + c = 0 .
So: a = − 1
Our required expression: a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 = a 2 0 1 3 = − 1