2016 is absolutely awesome 15

Algebra Level 4

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 \ne 0 .

Suppose that the following conditions hold:

  • f ( 1 ) = 0 f(1) = 0

  • the roots of g ( x ) g(x) are squares of the roots of f ( x ) f(x) .

Find the value of S = a 2016 + b 2016 + c 2016 S=a^{2016}+b^{2016}+ c^{2016} .


This is a part of the Set .


The answer is 3.

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2 solutions

Note that g ( 1 ) = f ( 1 ) = 0 g(1) = f(1) = 0 , so 1 1 is a root of both f ( x ) f(x) and g ( x ) g(x) .

Let p p and q q be the other two roots of f ( x ) f(x) , so p 2 p^2 and q 2 q^2 are the other two roots of g ( x ) g(x) .

We then get p q = c pq = -c and p 2 q 2 = a p^2q^2 = -a , so a = c 2 a = -c^2 .

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

Therefore b = c 4 b = c^4 .

Since f ( 1 ) = 0 f(1) = 0 we therefore get 1 + c c 2 + c 4 = 0 1 + c - c^2 + c^4 = 0 .

Factorising, we get ( c + 1 ) ( c 3 c 2 + 1 ) = 0 (c + 1)(c^3 - c^2 + 1) = 0 .

Note that c 3 c 2 + 1 = 0 c^3 - c^2 + 1 = 0 has no integer root and hence c = 1 , b = 1 , a = 1 c = -1, b = 1, a = -1 .

Therefore S = a 2016 + b 2016 + c 2016 = 3 S=a^{2016} + b^{2016} + c^{2016} = \boxed{3} .

Arihant Samar
Mar 11, 2016

Though Khang Nguyen Thanh's solution is the best,here is another approach which is a little TROLL and objective.

We know that any integer to the power 2016 is always positive.

If any of a , b , c a,b,c is either 2 \le-2 or 2 \ge2 then a 2016 + b 2016 + c 2016 = 2 2016 + s o m e t h i n g p o s i t i v e a^{2016}+b^{2016}+c^{2016}=2^{2016} + something positive

Since such an huge answer cannot be entered in Brilliant answer box,we now have to check for the values 0 , 1 , 1 0,1,-1 . But since c 0 c\ne0 , c 2016 = 1 c^{2016}=1 .

Hence this rules out 0 0 as an answer.Now the answers can only be 1 , 2 , 3 1,2,3 which can be easily done since we get 3 3 tries.

Thus when you enter 3 3 as your answer,you get CORRECT

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