A = cyc ∑ β γ + 1 6 α
If α , β , γ are roots of the equation x 3 − 7 x 2 + 5 x + 2 = 0 , then find A .
Also try the Part-2 .
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Last method was very innovative.
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Yep... :-)
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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)..... ;>
You are really Cool .. Never seen such variety of series based questions like the ones in your feed.
A = cyc ∑ β γ + 1 6 α = 6 cyc ∑ α α β γ + 1 α Vieta’s formula: α β γ = − 2 = 6 cyc ∑ α − 2 α 2 = 6 cyc ∑ α − 2 α 2 − 4 + 4 = 6 cyc ∑ α − 2 ( α − 2 ) ( α + 2 ) + 4 = 6 cyc ∑ ( α + 2 + α − 2 4 ) = 6 ( α + β + γ + 6 + 4 ( α − 2 1 + β − 2 1 + γ − 2 1 ) ) Vieta’s formula: α + β + γ = 7 = 6 ( 7 + 6 + 4 ( − ( 2 − α ) ( 2 − β ) ( 2 − γ ) α β + β γ + γ α − 4 ( α + β + γ ) + 1 2 ) ) Vieta’s: α β + β γ + γ α = 5 = 7 8 + 2 4 ( − ( − 8 ) 5 − 4 ( 7 ) + 1 2 ) As ( 2 − α ) ( 2 − β ) ( 2 − γ ) = 2 3 − 7 ( 2 2 ) + 5 ( 2 ) + 2 = − 8 = 4 5 None of the other options
Damn it. I forgot to multiply that six outside that cyclic summation , hence got answer as 7.5
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Yep I knew.... That's why I kept 7.5 as a option ... :-)
@Chew-Seong Cheong Sir does this method work for all Vieta's Problem Solving Questions ?
A solution that uses Newton's sums:
Note that for my solution, let α = a , β = b , γ = c
From Vieta's formula, we get
a + b + c = 7 a b + a c + b c = 5 a b c = − 2
A = cyc ∑ b c + 1 6 a = cyc ∑ a b c + a 6 a 2 = cyc ∑ a − 2 6 a 2 = a − 2 6 a 2 + b − 2 6 b 2 + c − 2 6 c 2 = ( a − 2 ) ( b − 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 b c − 2 ( a b + a c + b c ) + 4 ( a + b + c ) − 8 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 ) = − 2 − 2 ( 5 ) + 4 ( 7 ) − 8 6 a 2 ( a − 2 − 2 ( 7 − a ) + 4 ) + 6 b 2 ( b − 2 − 2 ( 7 − b ) + 4 ) + 6 c 2 ( c − 2 − 2 ( 7 − c ) + 4 ) = − 2 − 1 0 + 2 8 − 8 6 a 2 ( a − 2 − 1 0 + 2 a ) + 6 b 2 ( b − 2 − 1 0 + 2 b ) + 6 c 2 ( c − 2 − 1 0 + 2 c ) = 8 − 1 2 a − 6 0 a 2 + 1 2 a 3 − 1 2 b − 6 0 b 2 + 1 2 b 3 − 1 2 c − 6 0 c 2 + 1 2 c 3 = 8 1 2 ( a 3 + b 3 + c 3 ) − 6 0 ( a 2 + b 2 + c 2 ) − 1 2 ( a + b + c )
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 ) = 3 9 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 ) ( 3 9 ) − ( 5 ) ( 7 ) + 3 ( − 2 ) = 2 3 2
Substitute these values in:
A = 8 1 2 ( a 3 + b 3 + c 3 ) − 6 0 ( a 2 + b 2 + c 2 ) − 1 2 ( a + b + c ) = 8 1 2 ( 2 3 2 ) − 6 0 ( 3 9 ) − 1 2 ( 7 ) = 8 2 7 8 4 − 2 3 4 0 − 8 4 = 8 3 6 0 = 4 5
Therefore, the answer is 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 − 4 5 as one of the options on purpose
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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' :-)
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Now i will respect None of the above option since i marked -45 :P
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@Harsh Shrivastava – You fell for it...I nearly did too, I double checked
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@Hung Woei Neoh – Bravo... The trick worked..... ;-p
A very long solution but still... (+1) for your courage ;-)
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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!
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Done... A similar solution for that problem also exists .... Nice modification though ... Great work :-)
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)
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Then, I propose "None of the given options" instead. "None of the other options" sounds weird to me
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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)
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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
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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... :-)
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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Using Vieta's formula: cyc ∑ α = 7 , cyc ∑ α β = 5 , α β γ = − 2
A = 6 cyc ∑ ⎝ ⎜ ⎜ ⎛ − 2 α β γ + α α 2 ⎠ ⎟ ⎟ ⎞
= 6 cyc ∑ ⎝ ⎜ ⎜ ⎜ ⎛ α − 2 α 2 4 + ( α 2 − 4 ) ⎠ ⎟ ⎟ ⎟ ⎞
= 6 cyc ∑ ( α − 2 4 + α + 2 )
Now cyc ∑ α − 2 1 can be found using 3 m e t h o d s :
( 1 ) . Take LCM so that it finally comes out to be 4 cyc ∑ α − 2 cyc ∑ α β + α β γ − 8 cyc ∑ α β − 4 cyc ∑ α + 1 2 and just put values.
( 2 ) . Transform the polynomial by putting x = y 2 y + 1 and find its sum of roots using Vieta's formula.
( 3 ) . Write f ( x ) = ( x − α ) ( x − β ) ( x − γ ) , take ln and differentiate both sides to get : f ( x ) − f ′ ( x ) = cyc ∑ α − x 1 and just put x = 2 so that finally you'll get cyc ∑ α − 2 1 = 8 − 1 1 , so that:
A = 6 ( 4 × 8 − 1 1 + 7 + 6 ) = 4 5
So the answer is None of the other options .