Newtons polynomial

Algebra Level 5

The polynomial x 3 + M x N x^3+Mx-N has roots a , b a,b and c c , where M M and N N are constant integers satisfying the inequality 1 < M < N 1<|M| <|N| .

Given that a 5 + b 5 + c 5 = 90 a^5 +b^5+c^5=90 and a 6 + b 6 + c 6 = 227 a^6 + b^6 + c^6 = 227 , find the value of M 5 + N 5 M^5 + N^5 .


The answer is -59017.

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1 solution

Chew-Seong Cheong
Dec 26, 2015

We solve this using Vieta's formulas. Let P n = a n + b n + c n P_n = a^n+b^n+c^n , S 1 = a + b + c = 0 S_1 = a+b+c = 0 , S 2 = a b + b c + c a = M S_2 = ab+bc+ca = M and S 3 = a b c = N S_3 = abc=N . Then, we have:

\(\begin{array} {} P_2 = a^2 + b^2+c^2 = S_1P_1 - 2S_2 = (0)(0)-2M & = -2M \\ P_3 = a^3 + b^3+c^3 = 0-M(0)+3N & = 3N \\ P_4 = a^4 + b^4+c^4 = 0-M(-2M)+N(0) & = 2M^2 \\ P_5 = a^5 + b^5+c^5 = 0-M(3N)+N(-2M) = -5M = 90 & \Rightarrow MN = - 18 & ...(1) \\ P_6 = a^6 + b^6+c^6 = 0 -M(2M^2) + N(3N) = 227 & \Rightarrow -2M^3 + 3N^2 = 227 &...(2) \end{array} \)

Solving using equations (1) and (2), we found that M = 2 M=2 and N = 9 N=-9 .

Therefore, M 5 + N 5 = 2 5 + ( 9 ) 5 = 32 59049 = 59017 M^5+N^5 = 2^5 + (-9)^5 = 32-59049 = \boxed{-59017}

I did the exact same thing.

Anupam Nayak - 5 years, 5 months ago

I did same.

Nice problem.

Dev Sharma - 5 years, 5 months ago

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Thanx buddy

shivamani patil - 5 years, 5 months ago

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Great Problem :D

A Former Brilliant Member - 5 years, 5 months ago

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@A Former Brilliant Member Thax brother

shivamani patil - 5 years, 5 months ago

Your values of M and N are flipped.

Pi Han Goh - 5 years, 5 months ago

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Thanks, pal.

Chew-Seong Cheong - 5 years, 5 months ago

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