Let f ( x ) = x 3 + a x 2 + b x + c and g ( x ) = x 3 + b x 2 + c x + a , where a , b , c are integers with c = 0 . Suppose the following conditions hold :
f ( 1 ) = 0 .
The roots of g ( x ) are squares of roots of f ( x ) .
Find the value of a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 .
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Something is not quite right here... c 3 − c 2 + c in the last line should be c 3 − c 2 + 1 , I think.
And if c = a = − 1 , b = 0 , the answer would be ( − 1 ) + 0 + ( − 1 ) = − 2 .
Your solution is very good, but you just need to fix a typo. It should say b = 1 . And for anyone else reading this, it might help to add a step: p 2 + q 2 + 1 + 2 ( p q + q + p ) = − b + 2 b = b .
We know that both f ( 1 ) = 0 and g ( 1 ) = 0 because 1 2 = 1 . Substituting 1 into either polynomial gives the same result, relating a , b , and c : a + b + c + 1 = 0 . Then I use synthetic division to partially factor each polynomial as f ( x ) = ( x − 1 ) ( x 2 + ( a + 1 ) x + a + b + 1 ) and g ( x ) = ( x − 1 ) ( x 2 + ( b + 1 ) x + b + c + 1 ) . Using the relationship a + b + c + 1 = 0 , I can write g ( x ) = ( x − 1 ) ( x 2 + ( b + 1 ) x − a ) , eliminating the variable c . Let α and β be the other roots of f ( x ) --other than the known root x = 1 . Then through applying Vieta's formulas to both f ( x ) and g ( x ) , I get the four equations: α + β = − a − 1 , α β = a + b + 1 , α 2 + β 2 = − b − 1 , α 2 β 2 = − a . I first square the second equation, so that I can equate it with the fourth: α 2 β 2 = − a = a 2 + b 2 + 2 a b + 2 a + 2 b + 1 .Then I square the first, multiply the second by 2 , subtract the resulting equations, and equate it with the third equation to get: α 2 + β 2 = a 2 − 2 b − 1 = − b − 1 , which implies a 2 = b . Then after substituting a 2 = b into a 2 + b 2 + 2 a b + 3 a + 2 b + 1 = 0 , and simplifying, I get a 4 + 2 a 3 + 3 a 2 + 3 a + 1 = 0 . There is only one integer root of this polynomial (cf. https://brilliant.org/wiki/rational-root-theorem/), which leads to an answer of a = − 1 , b = 1 , c = − 1 . Hence, a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 = ( − 1 ) 2 0 1 3 + ( 1 ) 2 0 1 3 + ( − 1 ) 2 0 1 3 = − 1 .
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Note that g ( 1 ) = f ( 1 ) = 0 , so 1 is the root of both g ( x ) a n d f ( x ) .
Let p and q are other two roots of f(x), so p 2 and q 2 are other two roots of g ( x ) . We then get − c = p q and − a = p 2 q 2 , so a = − c 2 .
Also ( − a ) 2 = ( p + q + 1 ) 2 = p 2 + q 2 + 1 + 2 ( p q + q + p ) = b .
Therefore b = c 4 . Since f ( 1 ) = 0 , c + 1 − c 2 + c 4 = 0 . Factorizing, we get ( c + 1 ) ( c 3 − c 2 + 1 ) = 0 .
Note that c 3 − c 2 + 1 = 0 has no integer root hence, c = − 1 , b = 0 , a = − 1 . So, a 2 0 1 3 + b 2 0 1 3 + c 2 0 1 3 = − 1 .