If a , b and c are complex numbers that satisfy: a 2 + b 2 + c 2 = − 3 a 3 + b 3 + c 3 = − 4 6 a 4 + b 4 + c 4 = − 1 2 3 And it's known that the value of a + b + c is an integer.
Find ( a 1 0 + b 1 0 + c 1 0 ) m o d 1 0 0 0 .
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EDIT: Rating dropped by 73%. How many people solved this legidly?
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That's unfair, the rating of my problem dropped from 220 to 65 now.
Read Julian Poon's solution on how to get that a , b , c are roots of
0 = m 4 + 1 8 m 2 + 3 6 8 m + 7 6 5 = ( m + 5 ) ( m 3 − 5 m 2 + 4 3 m + 1 5 3 )
It's easy to see that 0 = m 3 − 5 m 2 + 4 3 m + 1 5 3 has no integer roots. However, it has at least one real root x and two complex roots y and z . Since x + y + z = 5 from Vieta's formulas, and none of a , b , c are integers, none of x + y , y + z , or z + x are integers. If we add the other root − 5 to any of these, they still won't be integers. Therefore, the only possible combination of these roots such that they sum to an integer is the roots of 0 = m 3 − 5 m 2 + 4 3 m + 1 5 3 . This means that a , b , c are the roots of 0 = m 3 − 5 m 2 + 4 3 m + 1 5 3 .
Now apply Newton's sums 1 0 times on the roots of this cubic equation to get the desired answer. There is no need to actually find the roots explicitly.
Did you mean that all the values of − ( a + b + c ) are roots of m 4 + 1 8 m 2 + 3 6 8 m + 7 6 5 ? Because a , b and c , for m = − 5 are roots of x 3 − 5 x 2 + 1 4 x − 1 3 .
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Im pretty surprised that after 2 hours of huge audience there isn't a single solution. I solved this quite a while ago so the solution does not have the small details
Let f ( n ) = a n + b n + c n and a , b and c be the roots of x 3 + m x 2 + n x + p
Using newton sums,
f ( 1 ) = − m f ( 2 ) = m 2 − 2 n = − 3 f ( 3 ) = − m 3 + 3 m n − 3 p = − 4 6 f ( 4 ) = m 4 − 4 m 2 n + 4 m p + 2 n 2 = − 1 2 3
It is pretty simple, now, just solve for m and the answer would be − m . Yeah... simple.
Using numerous substitutions, we get a Polynomial of degree 4
6 m 4 + 3 m 2 + 3 1 8 4 m + 1 2 7 . 5 = 0
So, solving this tediously, (I used Vieta's formula), we get 2 real and 2 complex solutions:
m = − 5
m = − 2 . 4 8 4 1 1 . . .
m = 3 . 7 4 2 0 6 . . . − 6 . 8 9 8 4 3 . . . i
m = 3 . 7 4 2 0 6 . . . + 6 . 8 9 8 4 3 . . . i
Since − m has to be an integer (stated), f ( 1 ) = 5
Now, using newton sums again,
f ( 1 ) = S 1 = 5 f ( 2 ) = S 1 2 − 2 S 2 = − 3 f ( 3 ) = S 1 f ( 2 ) − S 2 f ( 1 ) + 3 S 3 = − 4 6 f ( n ) = f ( 1 ) f ( n − 1 ) − S 2 f ( n − 2 ) + S 3 f ( n − 3 )
We can again tediously calculate that f ( 1 0 ) = − 1 0 4 8 3 8
Therefore, the answer is 1 6 2
Thanks Alan Enrique Ontiveros Salazar! This question's rating has dropped by 58% since it's peak.