Quinquennial Sum

Algebra Level 5

x 4 7 x 2 + 4 x 3 = 0 { x }^{ 4 }-{ 7x }^{ 2 }+4x-3=0

Find the sum of the fifth powers of the roots of the equation above.


The answer is -140.

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1 solution

Chew-Seong Cheong
Mar 10, 2015

The problem can be solved using Newton's Sum method.

Let the roots of the equation be a a , b b , c c and d d , then:

{ S 1 = a + b + c + d = 0 S 2 = a b + a c + a d + b c + b d + c d = 7 S 3 = a b c + a b d + a c d + b c d = 4 S 4 = a b c d = 3 \begin{cases} S_1 = a+b+c+d & = 0 \\ S_2 = ab+ac+ad+bc+bd+cd & = -7 \\ S_3 = abc+abd+acd+bcd & = -4 \\ S_4 = abcd & = -3 \end{cases}

And

{ P 1 = a + b + c + d = S 1 = 0 P 2 = a 2 + b 2 + c 2 + d 2 = S 1 P 1 2 S 2 = 14 P 3 = a 3 + b 3 + c 3 + d 3 = S 1 P 2 S 2 P 1 + 3 S 3 = 12 P 4 = a 4 + b 4 + c 4 + d 4 = S 1 P 3 S 2 P 2 + S 3 P 1 4 S 4 = 110 P 4 = a 5 + b 5 + c 5 + d 5 = S 1 P 4 S 2 P 3 + S 3 P 2 S 4 P 1 = 140 \begin{cases} P_1 = a+b+c+d & = S_1 & = 0 \\ P_2 = a^2+b^2+c^2+d^2 & = S_1P_1-2S_2 & = 14 \\ P_3 = a^3+b^3+c^3+d^3 & = S_1P_2-S_2P_1 + 3S_3 & = -12 \\ P_4 = a^4+b^4+c^4+d^4 & = S_1P_3-S_2P_2 + S_3P_1 - 4S_4 & = 110 \\ P_4 = a^5+b^5+c^5+d^5 & = S_1P_4-S_2P_3 + S_3P_2 - S_4P_1 & = \boxed{-140} \end{cases}

The calculations can be easily done with a spreadsheet as follows:

I remembered this answer, it's from Hall & Knight.

Satvik Golechha - 6 years, 3 months ago

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Most of his questions are from either "Hall & Night" or "A.I Prilepko"

@Satvik Golechha

Parth Lohomi - 6 years, 3 months ago

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Ahh! revealed your truth.So you see the answers and write here overrated at all the problems

Utkarsh Bansal - 6 years, 3 months ago

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@Utkarsh Bansal I have solved these problems earlier they are not level 5 levels , therefore I write overrated there.As you can see I have not written OVERRATED here,I have written overrated in only 2 to 3 problems (not all),So what is the meaning of arguing with me,Also IN the problems where i have written OVERRATED have come down from their particular level.


ALso try to post your "original" problems , that will increase your creativity.

¨ \ddot\smile

Also I am not the only one who says your problems overrated

see this

, this ,

this(calvin sir thinks it is overrated)

Parth Lohomi - 6 years, 3 months ago

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@Parth Lohomi So please post a solution for this problem @Parth Lohomi and if you think my problems are overrated please try to post solution of this

Utkarsh Bansal - 6 years, 3 months ago

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@Utkarsh Bansal Use newtons sum , Also as one studies for RMO these type of questions are practised by him/her > @Utkarsh Bansal I have not solved that problem yet then why are you asking me for a solution?? (you made the question right, then how had you solved it?)

Parth Lohomi - 6 years, 3 months ago

Why do such straight forward questions get to level 5 ?

Rohit Shah - 6 years, 3 months ago

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It is becoz your problem solving skill is good.Not all of us here has that ability.That's why its level 5

Utkarsh Bansal - 6 years, 3 months ago

Nice solution sir , Upvoted

Utkarsh Bansal - 6 years, 3 months ago

Newton's Sums are just so tedious. There must be another way!

Ryan Tamburrino - 6 years, 3 months ago

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You can multiply the equation by x and sum it in that case you only need P1 P2 and P3. That is not really tedious

Rohit Shah - 6 years, 3 months ago

Ryan, it can be easily done with a spreadsheet. I can even solve to P 50 P_{50} easily by just hold and drag to copy the formulas. I have edited my solution above.

Chew-Seong Cheong - 6 years, 3 months ago

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That's true. Spreadsheets are neat.

Ryan Tamburrino - 6 years, 3 months ago

We can Reduce the calculation a bit by writting x 5 = 7 x 3 4 x 2 + 3 x x^{5}=7x^{3}-4x^{2}+3x

Shubhendra Singh - 6 years, 2 months ago

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