What about this? -1

Algebra Level 5

A = cyc 6 α β γ + 1 \large \mathcal A =\displaystyle\sum_{\text{cyc}} \frac{6\alpha}{\beta\gamma+1}

If α , β , γ \alpha,\beta,\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 .


Also try the Part-2 .

15 44 90 7.5 None of the other options -45 3 0

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4 solutions

Rishabh Jain
Jun 23, 2016

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 = 6 cyc ( α 2 α β γ 2 + α ) \mathcal A=6\displaystyle\sum_{\text{cyc}}\left(\dfrac{\alpha^2}{\underbrace{\alpha\beta\gamma}_{-2}+\alpha}\right)

= 6 cyc ( α 2 4 + ( α 2 4 ) α 2 ) = 6\displaystyle\sum_{\text{cyc}}\left(\dfrac{\overbrace{\alpha^2}^{ \color{#D61F06}{4}+(\alpha^2-\color{#D61F06}{4})}}{\alpha-2}\right)

= 6 cyc ( 4 α 2 + α + 2 ) =6\displaystyle\sum_{\text{cyc}}\left(\dfrac{4}{\alpha-2}+\alpha+2\right)

Now cyc 1 α 2 \small{\displaystyle\sum_{\text{cyc}}\dfrac{1}{\alpha-2}} can be found using 3 m e t h o d s \large\color{#EC7300}{3~methods} :


( 1 ) . (1). Take LCM so that it finally comes out to be cyc α β 4 cyc α + 12 4 cyc α 2 cyc α β + α β γ 8 \small{\dfrac{\displaystyle\sum_{\text{cyc}}\alpha\beta-4\displaystyle\sum_{\text{cyc}}\alpha+12}{4\displaystyle\sum_{\text{cyc}}\alpha-2\displaystyle\sum_{\text{cyc}}\alpha\beta+ \alpha\beta\gamma-8}} and just put values.

( 2 ) . (2). Transform the polynomial by putting x = 2 y + 1 y x=\dfrac{2y+1}{y} and find its sum of roots using Vieta's formula.

( 3 ) . (3). Write f ( x ) = ( x α ) ( x β ) ( x γ f(x)=(x-\alpha)(x-\beta)(x-\gamma ) , take ln \ln and differentiate both sides to get : f ( x ) f ( x ) = cyc 1 α x \small{\dfrac{-f'(x)}{f(x)}=\displaystyle\sum_{\text{cyc}}\dfrac1{\alpha-x}} and just put x = 2 x=2 so that finally you'll get cyc 1 α 2 = 11 8 \displaystyle\sum_{\text{cyc}}\dfrac{1}{\alpha-2}=\dfrac{-11}{8} , so that:


A = 6 ( 4 × 11 8 + 7 + 6 ) = 45 \mathcal A=6\left(4\times\dfrac{-11}8+7+6\right)=\boxed{45}

So the answer is None of the other options \color{#3D99F6}{\boxed{\text{None of the other options}}} .

Last method was very innovative.

Anik Mandal - 4 years, 11 months ago

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Yep... :-)

Rishabh Jain - 4 years, 11 months ago

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@Rishabh Cool Can u please tell me from where i can learn Vieta's Problem Solving ??? Even though I got this question correct I didn't get 45 I got 36 (Lol)..... ;>

abc xyz - 4 years, 11 months ago

You are really Cool .. Never seen such variety of series based questions like the ones in your feed.

Aniruddha Bagchi - 3 years, 7 months ago
Chew-Seong Cheong
Jun 23, 2016

A = cyc 6 α β γ + 1 = 6 cyc α α β γ α + 1 Vieta’s formula: α β γ = 2 = 6 cyc α 2 α 2 = 6 cyc α 2 4 + 4 α 2 = 6 cyc ( α 2 ) ( α + 2 ) + 4 α 2 = 6 cyc ( α + 2 + 4 α 2 ) = 6 ( α + β + γ + 6 + 4 ( 1 α 2 + 1 β 2 + 1 γ 2 ) ) Vieta’s formula: α + β + γ = 7 = 6 ( 7 + 6 + 4 ( α β + β γ + γ α 4 ( α + β + γ ) + 12 ( 2 α ) ( 2 β ) ( 2 γ ) ) ) Vieta’s: α β + β γ + γ α = 5 = 78 + 24 ( 5 4 ( 7 ) + 12 ( 8 ) ) As ( 2 α ) ( 2 β ) ( 2 γ ) = 2 3 7 ( 2 2 ) + 5 ( 2 ) + 2 = 8 = 45 None of the other options \begin{aligned} \mathcal A & = \sum_\text{cyc} \frac {6\alpha}{\beta \gamma +1} \\ & = 6 \sum_\text{cyc} \frac \alpha {\frac {\color{#3D99F6}{\alpha \beta \gamma}} {\alpha} +1} \quad \quad \small \color{#3D99F6}{\text{Vieta's formula: }\alpha \beta \gamma = -2 } \\ & = 6 \sum_\text{cyc} \frac {\alpha^2} {\alpha -2} \\ & = 6 \sum_\text{cyc} \frac {\alpha^2-4+4} {\alpha -2} \\ & = 6 \sum_\text{cyc} \frac {(\alpha-2)(\alpha+2)+4} {\alpha -2} \\ & = 6 \sum_\text{cyc} \left(\alpha+2 + \frac 4 {\alpha -2}\right) \\ & = 6\left(\color{#3D99F6}{\alpha + \beta + \gamma} + 6 + 4\left(\frac 1 {\alpha -2} + \frac 1 {\beta -2} + \frac 1 {\gamma -2} \right) \right) \quad \quad \small \color{#3D99F6}{\text{Vieta's formula: }\alpha+\beta+\gamma = 7} \\ & = 6\left(\color{#3D99F6}{7} + 6 + 4\left(\frac {\color{#3D99F6}{\alpha \beta + \beta \gamma + \gamma \alpha} -4(\color{#3D99F6}{\alpha + \beta + \gamma})+12}{-\color{#D61F06}{(2-\alpha)(2-\beta)(2-\gamma)}} \right) \right) \quad \quad \small \color{#3D99F6}{\text{Vieta's: }\alpha \beta + \beta \gamma + \gamma \alpha = 5} \\ & = 78 + 24\left(\frac {\color{#3D99F6}{5} -4(\color{#3D99F6}{7})+12}{-\color{#D61F06}{(-8)}} \right) \quad \quad \small \color{#D61F06}{\text{As }(2-\alpha)(2-\beta)(2-\gamma) = 2^3 -7(2^2) +5(2)+2 = -8} \\ & = 45 \quad \boxed{\text{None of the other options}} \end{aligned}

Damn it. I forgot to multiply that six outside that cyclic summation , hence got answer as 7.5

Aakash Khandelwal - 4 years, 11 months ago

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Yep I knew.... That's why I kept 7.5 as a option ... :-)

Rishabh Jain - 4 years, 11 months ago

@Chew-Seong Cheong Sir does this method work for all Vieta's Problem Solving Questions ?

abc xyz - 4 years, 11 months ago

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Yes, it should be.

Chew-Seong Cheong - 4 years, 11 months ago
Hung Woei Neoh
Jun 25, 2016

A solution that uses Newton's sums:

Note that for my solution, 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 6 a b c + 1 = cyc 6 a 2 a b c + a = cyc 6 a 2 a 2 = 6 a 2 a 2 + 6 b 2 b 2 + 6 c 2 c 2 = 6 a 2 ( b 2 ) ( c 2 ) + 6 b 2 ( a 2 ) ( c 2 ) + 6 c 2 ( a 2 ) ( b 2 ) ( a 2 ) ( b 2 ) ( c 2 ) = 6 a 2 ( b c 2 ( b + c ) + 4 ) + 6 b 2 ( a c 2 ( a + c ) + 4 ) + 6 c 2 ( a b 2 ( a + b ) + 4 ) a b c 2 ( a b + a c + b c ) + 4 ( a + b + c ) 8 = 6 a 2 ( 2 a 2 ( 7 a ) + 4 ) + 6 b 2 ( 2 b 2 ( 7 b ) + 4 ) + 6 c 2 ( 2 c 2 ( 7 c ) + 4 ) 2 2 ( 5 ) + 4 ( 7 ) 8 = 6 a 2 ( 2 a 10 + 2 a ) + 6 b 2 ( 2 b 10 + 2 b ) + 6 c 2 ( 2 c 10 + 2 c ) 2 10 + 28 8 = 12 a 60 a 2 + 12 a 3 12 b 60 b 2 + 12 b 3 12 c 60 c 2 + 12 c 3 8 = 12 ( a 3 + b 3 + c 3 ) 60 ( a 2 + b 2 + c 2 ) 12 ( a + b + c ) 8 \mathcal A = \displaystyle \sum_{\text{cyc}} \dfrac{6a}{bc+1}\\ =\displaystyle \sum_{\text{cyc}} \dfrac{6a^2}{\color{#EC7300}{abc}+a}\\ =\displaystyle \sum_{\text{cyc}} \dfrac{6a^2}{a-2}\\ =\dfrac{6a^2}{a-2}+\dfrac{6b^2}{b-2}+\dfrac{6c^2}{c-2}\\ =\dfrac{6a^2(b-2)(c-2) + 6b^2(a-2)(c-2)+6c^2(a-2)(b-2)}{(a-2)(b-2)(c-2)}\\ =\dfrac{6a^2\big(\color{#EC7300}{bc} - 2(\color{#3D99F6}{b+c})+4\big)+6b^2\big(\color{#EC7300}{ac} - 2(\color{#3D99F6}{a+c})+4\big)+6c^2\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{6a^2\big(\color{#EC7300}{\frac{-2}{a}} - 2(\color{#3D99F6}{7-a})+4\big)+6b^2\big(\color{#EC7300}{\frac{-2}{b}} - 2(\color{#3D99F6}{7-b})+4\big)+6c^2\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{6a^2(\frac{-2}{a}-10+2a)+6b^2(\frac{-2}{b}-10+2b)+6c^2(\frac{-2}{c}-10+2c)}{-2-10+28 -8}\\ =\dfrac{-12a-60a^2+12a^3-12b-60b^2+12b^3-12c-60c^2+12c^3}{8}\\ =\dfrac{12(\color{#69047E}{a^3+b^3+c^3})-60(\color{#20A900}{a^2+b^2+c^2})-12(\color{#3D99F6}{a+b+c})}{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 \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}

Substitute these values in:

A = 12 ( a 3 + b 3 + c 3 ) 60 ( a 2 + b 2 + c 2 ) 12 ( a + b + c ) 8 = 12 ( 232 ) 60 ( 39 ) 12 ( 7 ) 8 = 2784 2340 84 8 = 360 8 = 45 \mathcal A = \dfrac{12(\color{#69047E}{a^3+b^3+c^3})-60(\color{#20A900}{a^2+b^2+c^2})-12(\color{#3D99F6}{a+b+c})}{8}\\ =\dfrac{12(\color{#69047E}{232})-60(\color{#20A900}{39})-12(\color{#3D99F6}{7})}{8}\\ =\dfrac{2784-2340-84}{8}\\ =\dfrac{360}{8}\\ =45

Therefore, the answer is None of the other options \boxed{\text{None of the other options}} (Though I'd nitpick and say that this is grammatically incorrect, it should be "None of the options above" instead)

Something tells me that you made this an MCQ, and placed 45 -45 as one of the options on purpose

Hung Woei Neoh - 4 years, 11 months ago

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Obviously each and every option has its own story and has been carefully placed to trick those who don't give proper respect to 'None of the other options' :-)

Rishabh Jain - 4 years, 11 months ago

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Now i will respect None of the above option since i marked -45 :P

Harsh Shrivastava - 4 years, 11 months ago

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@Harsh Shrivastava You fell for it...I nearly did too, I double checked

Hung Woei Neoh - 4 years, 11 months ago

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@Hung Woei Neoh Bravo... The trick worked..... ;-p

Rishabh Jain - 4 years, 11 months ago

A very long solution but still... (+1) for your courage ;-)

Rishabh Jain - 4 years, 11 months ago

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This is the actual method I used to solve it...never thought that I could use Newton's sums this way too...

Anyway, this solution isn't really long for me. I've written longer solutions. Though the color formatting took quite an amount of time

And besides, this solution and your question inspired me ! So I should thank you instead!

Hung Woei Neoh - 4 years, 11 months ago

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Done... A similar solution for that problem also exists .... Nice modification though ... Great work :-)

Rishabh Jain - 4 years, 11 months ago

No I think None of the others is most suitable and how could we use above since we don't know remaining options will be above this option ( If you get what I mean)

Rishabh Jain - 4 years, 11 months ago

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Then, I propose "None of the given options" instead. "None of the other options" sounds weird to me

Hung Woei Neoh - 4 years, 11 months ago

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Then also its not correct.... Since actually you are choosing "None of the given options" intead of the fact that you are selecting a option which says not even one of the given options are correct ( I know it sounds weird but think about it)

Rishabh Jain - 4 years, 11 months ago

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@Rishabh Jain Now that you mentioned it...hmmm....

I think that both of us are thinking too much about this XD

Hung Woei Neoh - 4 years, 11 months ago

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@Hung Woei Neoh This has been already well thought over on brilliant.... And "None of the others" is the most accepted... :-)

Rishabh Jain - 4 years, 11 months ago
Jus Jaisinghani
Jun 25, 2016

Took lcm multiplied the brackets and then substituded using vieta's formula was a bit long method but i got 45 as ans

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