What about this? 2

Algebra Level 5

A = cyc α 3 β γ + 1 \large \mathcal A =\displaystyle\sum_{\text{cyc}} \frac{\alpha^3}{\beta\gamma+1}

If α , β \alpha,\beta and γ \gamma are roots of the equation x 3 7 x 2 + 5 x + 2 = 0 x^3-7x^2+5x+2=0 , then find A \mathcal A .


Try the Part-1 and this problem first.


The answer is 340.

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2 solutions

Rishabh Jain
Jun 29, 2016

Relevant wiki: Vieta's Formula Problem Solving - Intermediate

Using Vieta's formula: cyc α = 7 , cyc α β = 5 , α β γ = 2 \displaystyle\sum_{\text{cyc}}\alpha=7,\displaystyle\sum_{\text{cyc}}\alpha\beta=5,\alpha\beta\gamma=-2

A = cyc ( α 4 α β γ 2 + α ) \mathcal A=\displaystyle\sum_{\text{cyc}}\left(\dfrac{\alpha^4}{\underbrace{\alpha\beta\gamma}_{-2}+\alpha}\right)

= cyc ( α 4 16 + ( α 4 16 ) α 2 ) = \displaystyle\sum_{\text{cyc}}\left(\dfrac{\overbrace{\alpha^4}^{ \color{#EC7300}{16}+(\alpha^4-\color{#EC7300}{16})}}{\alpha-2}\right)

= cyc ( 16 α 2 + ( α 2 ) ( α + 2 ) ( α 2 + 4 ) α 4 16 α 2 ) =\displaystyle\sum_{\text{cyc}}\left(\dfrac{16}{\alpha-2}+\dfrac{\overbrace{\cancel{(\alpha-2)}(\alpha+2)(\alpha^2+4)}^{\alpha^4-16}}{\cancel{\alpha-2}}\right)

Now cyc 1 α 2 \small{\displaystyle\sum_{\text{cyc}}\dfrac{1}{\alpha-2}} can be calculated using three ways which finally comes out to be 11 8 \dfrac{-11}8 . Now let's calculate second expression i.e cyc ( α 2 + 4 ) ( α + 2 ) = cyc α 3 + 2 α 2 + 4 α + 8 = cyc ( 9 α 2 α + 6 ) α 3 7 α 2 + 5 α + 2 = 0 = 9 ( ( cyc α ) 2 2 cyc α β ) 9 ( 49 10 ) = 351 7 + 6 × 3 = 362 \begin{aligned}&\displaystyle\sum_{\text{cyc}}(\alpha^2+4)(\alpha+2)\\=&\displaystyle\sum_{\text{cyc}}\alpha^3+2\alpha^2+4\alpha+8\\=&\displaystyle\sum_{\text{cyc}}\left(\color{#D61F06}{9\alpha^2}-\color{#20A900}{\alpha}+\color{#3D99F6}{6}\right)~~~\small{\because \alpha^3-7\alpha^2+5\alpha+2=0}\\=&\underbrace{\color{#D61F06}{9\left(\left(\displaystyle\sum_{\text{cyc}}\alpha\right)^2-2\displaystyle\sum_{\text{cyc}}\alpha\beta\right)}}_{\large 9(49-10)=351}-\color{#20A900}{7}+\color{#3D99F6}{6\times 3}\\=&362\end{aligned}

Hence,

A = 16 ( 11 8 ) + 362 \mathcal A=16\left(\dfrac{-11}{8}\right)+362 = 340 \huge =\boxed{340}

Your link to the "three ways" redirects to the mobile site

Btw, great solution!

Hung Woei Neoh - 4 years, 11 months ago
Hung Woei Neoh
Jun 29, 2016

A solution that uses Newton's sums:

For easy typing purposes, let α = a , β = b , γ = c \alpha = a,\;\beta=b,\;\gamma=c

From Vieta's formula, we get

a + b + c = 7 a b + a c + b c = 5 a b c = 2 \color{#3D99F6}{a+b+c = 7}\\ \color{#D61F06}{ab+ac+bc = 5}\\ \color{#EC7300}{abc = -2}

A = cyc a 3 b c + 1 = cyc a 4 a b c + a = cyc a 4 a 2 = a 4 a 2 + b 4 b 2 + c 4 c 2 = a 4 ( b 2 ) ( c 2 ) + b 4 ( a 2 ) ( c 2 ) + c 4 ( a 2 ) ( b 2 ) ( a 2 ) ( b 2 ) ( c 2 ) = a 4 ( b c 2 ( b + c ) + 4 ) + b 4 ( a c 2 ( a + c ) + 4 ) + c 4 ( a b 2 ( a + b ) + 4 ) a b c 2 ( a b + a c + b c ) + 4 ( a + b + c ) 8 = a 4 ( 2 a 2 ( 7 a ) + 4 ) + b 4 ( 2 b 2 ( 7 b ) + 4 ) + c 4 ( 2 c 2 ( 7 c ) + 4 ) 2 2 ( 5 ) + 4 ( 7 ) 8 = a 4 ( 2 a 10 + 2 a ) + b 4 ( 2 b 10 + 2 b ) + c 4 ( 2 c 10 + 2 c ) 2 10 + 28 8 = 2 a 3 10 a 4 + 2 a 5 2 b 3 10 b 4 + 2 b 5 2 c 3 10 c 4 + 2 c 5 8 = 2 ( a 5 + b 5 + c 5 ) 10 ( a 4 + b 4 + c 4 ) 2 ( a 3 + b 3 + c 3 ) 8 \mathcal A = \displaystyle \sum_{\text{cyc}} \dfrac{a^3}{bc+1}\\ =\displaystyle \sum_{\text{cyc}} \dfrac{a^4}{\color{#EC7300}{abc}+a}\\ =\displaystyle \sum_{\text{cyc}} \dfrac{a^4}{a-2}\\ =\dfrac{a^4}{a-2}+\dfrac{b^4}{b-2}+\dfrac{c^4}{c-2}\\ =\dfrac{a^4(b-2)(c-2) + b^4(a-2)(c-2)+c^4(a-2)(b-2)}{(a-2)(b-2)(c-2)}\\ =\dfrac{a^4\big(\color{#EC7300}{bc} - 2(\color{#3D99F6}{b+c})+4\big)+b^4\big(\color{#EC7300}{ac} - 2(\color{#3D99F6}{a+c})+4\big)+c^4\big(\color{#EC7300}{ab} - 2(\color{#3D99F6}{a+b})+4\big)}{\color{#EC7300}{abc} -2(\color{#D61F06}{ab+ac+bc})+4(\color{#3D99F6}{a+b+c}) -8}\\ =\dfrac{a^4\big(\color{#EC7300}{\frac{-2}{a}} - 2(\color{#3D99F6}{7-a})+4\big)+b^4\big(\color{#EC7300}{\frac{-2}{b}} - 2(\color{#3D99F6}{7-b})+4\big)+c^4\big(\color{#EC7300}{\frac{-2}{c}} - 2(\color{#3D99F6}{7-c})+4\big)}{\color{#EC7300}{-2} -2(\color{#D61F06}{5})+4(\color{#3D99F6}{7}) -8}\\ =\dfrac{a^4(\frac{-2}{a}-10+2a)+b^4(\frac{-2}{b}-10+2b)+c^4(\frac{-2}{c}-10+2c)}{-2-10+28 -8}\\ =\dfrac{-2a^3-10a^4+2a^5-2b^3-10b^4+2b^5-2c^3-10c^4+2c^5}{8}\\ =\dfrac{2(\color{teal}{a^5+b^5+c^5})-10(\color{magenta}{a^4+b^4+c^4})-2(\color{#69047E}{a^3+b^3+c^3})}{8}

Now, we apply Newton's sums:

a + b + c = 7 a 2 + b 2 + c 2 = ( a + b + c ) 2 2 ( a b + a c + b c ) = ( 7 ) 2 2 ( 5 ) = 39 a 3 + b 3 + c 3 = ( a + b + c ) ( a 2 + b 2 + c 2 ) ( a b + a c + b c ) ( a + b + c ) + 3 ( a b c ) = ( 7 ) ( 39 ) ( 5 ) ( 7 ) + 3 ( 2 ) = 232 a 4 + b 4 + c 4 = ( a + b + c ) ( a 3 + b 3 + c 3 ) ( a b + a c + b c ) ( a 2 + b 2 + c 2 ) + a b c ( a + b + c ) = ( 7 ) ( 232 ) ( 5 ) ( 39 ) + ( 2 ) ( 7 ) = 1415 a 5 + b 5 + c 5 = ( a + b + c ) ( a 4 + b 4 + c 4 ) ( a b + a c + b c ) ( a 3 + b 3 + c 3 ) + a b c ( a 2 + b 2 + c 2 ) = ( 7 ) ( 1415 ) ( 5 ) ( 232 ) + ( 2 ) ( 39 ) = 8667 \color{#3D99F6}{a+b+c = 7}\\ \color{#20A900}{a^2+b^2+c^2} = (\color{#3D99F6}{a+b+c})^2 - 2(\color{#D61F06}{ab+ac+bc}) = (\color{#3D99F6}{7})^2 - 2(\color{#D61F06}{5}) = \color{#20A900}{39}\\ \color{#69047E}{a^3+b^3+c^3} = (\color{#3D99F6}{a+b+c})(\color{#20A900}{a^2+b^2+c^2}) - (\color{#D61F06}{ab+ac+bc})(\color{#3D99F6}{a+b+c}) + 3(\color{#EC7300}{abc}) = (\color{#3D99F6}{7})(\color{#20A900}{39}) - (\color{#D61F06}{5})(\color{#3D99F6}{7}) + 3(\color{#EC7300}{-2}) =\color{#69047E}{232}\\ \color{magenta}{a^4+b^4+c^4} = (\color{#3D99F6}{a+b+c})(\color{#69047E}{a^3+b^3+c^3}) - (\color{#D61F06}{ab+ac+bc})(\color{#20A900}{a^2+b^2+c^2}) + \color{#EC7300}{abc}(\color{#3D99F6}{a+b+c})\\ = (\color{#3D99F6}{7})(\color{#69047E}{232}) - (\color{#D61F06}{5})(\color{#20A900}{39}) + (\color{#EC7300}{-2})(\color{#3D99F6}{7}) =\color{magenta}{1415}\\ \color{teal}{a^5+b^5+c^5}= (\color{#3D99F6}{a+b+c})(\color{magenta}{a^4+b^4+c^4}) - (\color{#D61F06}{ab+ac+bc})(\color{#69047E}{a^3+b^3+c^3}) + \color{#EC7300}{abc}(\color{#20A900}{a^2+b^2+c^2})\\ = (\color{#3D99F6}{7})(\color{magenta}{1415}) - (\color{#D61F06}{5})(\color{#69047E}{232}) + (\color{#EC7300}{-2})(\color{#20A900}{39}) =\color{teal}{8667}

Substitute these values in:

A = 2 ( a 5 + b 5 + c 5 ) 10 ( a 4 + b 4 + c 4 ) 2 ( a 3 + b 3 + c 3 ) 8 = 2 ( 8667 ) 10 ( 1415 ) 2 ( 232 ) 8 = 17334 14150 464 8 = 2720 8 = 340 \mathcal A = \dfrac{2(\color{teal}{a^5+b^5+c^5})-10(\color{magenta}{a^4+b^4+c^4})-2(\color{#69047E}{a^3+b^3+c^3})}{8}\\ =\dfrac{2(\color{teal}{8667})-10(\color{magenta}{1415})-2(\color{#69047E}{232})}{8}\\ =\dfrac{17334-14150-464}{8}\\ =\dfrac{2720}{8}\\ =\boxed{340}

Nice... (+1)... I knew you would definitely post a solution... :-) PS:- I couldn't scroll your( and many of the solutions) on my mobile after these changes on brilliant.... Maybe you can use \ [ ...\ ] for each line next time for proper rendering... :-)

Rishabh Jain - 4 years, 11 months ago

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Thanks. When I saw the question, I knew it was solution-writing time XD

Now that you mention it, I also can't see the last few digits for a 5 + b 5 + c 5 a^5+b^5+c^5 on my pc...lemme edit it

Hung Woei Neoh - 4 years, 11 months ago

you don't like colors, do you? I say it because I have a doubt about your solution: weren't there more colors for using in latex?... :-----------------------------------------), Ohhhh, you didn't use yellow, what a horrible solution, you have to fix it...

Guillermo Templado - 4 years, 11 months ago

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Yellow is too bright...

Hung Woei Neoh - 4 years, 11 months ago

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I think for this exactly reason you should add it... if you don't do it, your solution will be too dark...

Guillermo Templado - 4 years, 11 months ago

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@Guillermo Templado I refuse :P

Hung Woei Neoh - 4 years, 11 months ago

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@Hung Woei Neoh haha, good, I was joking... Colorful solution, I have not read it but (+1)...

Guillermo Templado - 4 years, 11 months ago

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