Prakkash's polynomial

Algebra Level 3

Suppose α , β , \alpha, \beta, and γ \gamma are the roots of polynomial x 3 + p x 2 + q x + 2 = 0 x^{3} + px^{2} + qx + 2 = 0 and that α β + 1 = 0 \alpha \beta + 1 = 0 . What is the value of 2 p + q + 100 2p + q + 100 ?


This problem is shared by Prakkash M , who came across it in his Mathematical Olympiad Training.


The answer is 95.

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18 solutions

Kevin Fei
Aug 18, 2013

From Vieta's Theorem, or by expanding ( x α ) ( x β ) ( x γ ) = x 3 + p x 2 + q x + 2 (x - \alpha)(x - \beta)(x - \gamma) = x^{3}+px^{2}+qx+2 , we get α β γ = 2 -\alpha\beta\gamma = 2 .

α β + 1 = 0 α β = 1 \alpha\beta+1=0\Rightarrow\alpha\beta=-1 , we find γ = 2 \gamma=2 by substituting in the above equation.

γ = 2 \gamma=2 is a root, so

2 3 + 2 2 p + 2 q + 2 = 0 2^{3}+2^{2}p+2q+2=0

4 p + 2 q + 10 = 0 4p+2q+10=0

2 p + q + 5 = 0 2p+q+5=0

2 p + q + 100 = 95 2p+q+100=95

Moderator note:

Nicely done!

Nicely done! I'm voting you up!

Mursalin Habib - 7 years, 9 months ago

Very cool, I didn't think of using f ( 2 ) = 0 f(2) = 0

Jiayang Zhao - 7 years, 9 months ago

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Me too...:D

Muhammad Sofyan - 6 years, 8 months ago

Thanks guys! :D

Kevin Fei - 7 years, 9 months ago

This is how I solved it :)

Jordan Osborn - 7 years, 9 months ago

Solved by same method !

Priyansh Sangule - 7 years, 9 months ago
Michael Tang
Aug 19, 2013

By Vieta's Formulas, the product of the roots is α β γ = 2. \alpha\beta\gamma = -2. We know that α β = 1 , \alpha\beta = -1, so γ = 2. \gamma = 2. Since 2 2 is a root of the polynomial, we have 2 3 + p ( 2 2 ) + q ( 2 ) + 2 = 0 , 2^3 + p(2^2) + q(2) + 2 = 0, or 4 p + 2 q = 10. 4p + 2q = -10. Dividing through by 2 2 gives 2 p + q = 5. 2p+q=-5. Thus, 2 p + q + 100 = 95 . 2p+q+100 = \boxed{95}.

Moderator note:

Nicely done!

It seems like most solutions followed the same pattern: Vieta's theorem, then plugging in 2 2 . Sadly, I don't know any country where Vieta's theorem for equation of degree more than two is part of standard school curriculum. On the bright side, this classical result is so well known that a serious student of mathematics simply cannot avoid stumbling at it at some point :)

but how can I know that the product of the roots is -2?

Ewerton Cassiano - 7 years, 9 months ago

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That's what Vieta's Formulas say. Try letting x 3 + p x 2 + q x + 2 = ( x α ) ( x β ) ( x γ ) x^3 + px^2 + qx + 2 = (x-\alpha)(x-\beta)(x-\gamma) and expand; then you can compare coefficients.

Michael Tang - 7 years, 9 months ago

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Thank you

Ewerton Cassiano - 7 years, 9 months ago

Although I knew what this formula might be about..but we never learned what formula in specific says that! Good to know a 13 yr old American knows this.Were you taught this in school? Because we in India, wouldn't get across this name during schooling....probably ever.

Ariba Siddiqui - 7 years, 9 months ago

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No, schools here don't really teach problem solving :P I take classes on the website Art of Problem Solving (AoPS) - aops.com.

Michael Tang - 7 years, 9 months ago

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Yes, AoPS is a great website. Not everyone can afford their online classes, but they have a lot of useful free information too.

Alexander Borisov - 7 years, 9 months ago

Cool.

Ariba Siddiqui - 7 years, 9 months ago

Hey Michael, do you play ftw? If you do, what is your rating?

revanth gumpu - 7 years, 9 months ago

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@Revanth Gumpu It's around the 1500s. I don't play much anymore though.

Michael Tang - 7 years, 9 months ago

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@Michael Tang What is your username? Mine is kevjumba

revanth gumpu - 7 years, 9 months ago

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@Revanth Gumpu I'll send you a PM on AoPS.

Michael Tang - 7 years, 9 months ago

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@Michael Tang kk

revanth gumpu - 7 years, 9 months ago

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@Revanth Gumpu Dude, I know MSTang, wow, i didnt know that was you.

revanth gumpu - 7 years, 9 months ago
Raymond Lin
Aug 19, 2013

By Vieta's Formulas, p = α + β + γ p=\alpha+\beta+\gamma , q = α β + α γ + β γ q=\alpha \beta+ \alpha \gamma + \beta \gamma , and 2 = α β γ 2=\alpha \beta \gamma .

Since α β + 1 = 0 \alpha \beta + 1 =0 , or α β = 1 \alpha \beta = -1 , we have 2 = ( 1 ) γ 2= (-1) \gamma , so γ = 2 \gamma = -2 .

From this, we have that p = α + β 2 p=\alpha+\beta-2 and q = α β 2 ( α + β ) q=\alpha \beta - 2(\alpha + \beta) , so 2 p + q + 100 = 2 ( α + β 2 ) + α β 2 ( α + β ) + 100 2p+q+100 = 2(\alpha+\beta-2) + \alpha \beta - 2(\alpha + \beta)+100 .

Simplifying and plugging in our value for α β \alpha\beta , we get our answer to be 2 ( 2 ) 1 + 100 = 95 2(-2)-1+100 = \fbox{95} .

Toshi Parmar
Aug 20, 2013

Its given that- αβ+1=0 => αβ= -1 Now, product of roots oof a cubic equals negative of coefficient of x^{0} divided by coefficient of x^{3}. Thus, αβγ= -(2) i.e. (-1)γ= -2 => γ=2 since its a solution of the cubic, 2^{3}+ 2^{2}p +q*2 +2 =0 => 2p+q= -5 so, 2p+q+100 = -5+100 = 95 :)

Jia Zheng Chai
Aug 19, 2013

By using Vieta's Formula,we know that

α + β + γ = p \alpha+\beta+\gamma=-p

α β + α γ + β γ = q \alpha\beta+\alpha\gamma+\beta\gamma=q

α β γ = 2 \alpha\beta\gamma=-2

In addition,we know that α β = 1 γ = 2 \alpha\beta=-1 \Rightarrow \gamma=2

By substituting the value,we get

α + β = p 2 2 α + 2 β = 2 p 4 \alpha+\beta=-p-2 \Rightarrow 2\alpha+2\beta=-2p-4

and

2 α + 2 β = q + 1 2\alpha+2\beta=q+1

By equating the two equations,

2 p 4 = q + 1 2 p + q = 5 2 p + q + 100 = 95 -2p-4=q+1\Rightarrow2p+q=-5\Rightarrow2p+q+100=95

Jonathon Capps
Aug 18, 2013

Expanding and factoring gives:

p = γ β α p = -γ-β-α

q = β γ + α γ + α β q = βγ+αγ+αβ

α β γ = 2 -αβγ=2

Given α β + 1 = 0 αβ+1=0 then α β = 1 αβ=-1 and γ = 2 γ=2

2 p + q + 100 2p+q+100

= 2 ( 2 β α ) + 2 β + 2 α + α β + 100 =2(-2-β-α)+2β+2α+αβ+100

= α β + 96 = 95 =αβ+96=95

Given that α β + 1 = 0 \alpha \beta + 1 = 0 or α β = 1 \alpha \beta = - 1 and using Vieta's formula, we have:

{ α β = 1 . . . ( 0 ) α + β + γ = p . . . ( 1 ) α β + β γ + γ α = q . . . ( 2 ) α β γ = 2 . . . ( 3 ) \begin{cases} \alpha \beta = - 1 &...(0) \\ \alpha + \beta + \gamma = - p &...(1) \\ \alpha \beta + \beta \gamma + \gamma \alpha = q &...(2) \\ \alpha \beta \gamma = - 2 &...(3) \end{cases}

( 0 ) ( 3 ) : α β γ = ( 1 ) γ = 2 γ = 2 ( 1 ) : α + β + γ = α + β + 2 = p α + β = p 2 ( 2 ) : α β + β γ + γ α = 1 + 2 ( α + β ) = 1 + 2 ( p 2 ) = q q = 5 2 p \begin{array} {rll} (0) \to (3): & \color{#3D99F6}{\alpha \beta} \gamma = \color{#3D99F6}{(-1)} \gamma =-2 & \Rightarrow \gamma = 2 \\ (1): & \alpha + \beta + \color{#3D99F6}{\gamma} = \alpha + \beta + \color{#3D99F6}{2} = - p & \Rightarrow \alpha + \beta = - p - 2 \\ (2): & \alpha \beta + \beta \gamma + \gamma \alpha = -1 + 2(\color{#3D99F6}{\alpha + \beta}) = - 1 + 2(-p-2) = q & \Rightarrow q = -5 - 2p \end{array}

q = 5 2 p 2 p + q = 5 2 p + q + 100 = 100 5 = 95 \begin{aligned} q & = -5 - 2p \\ \Rightarrow 2p + q & = -5 \\ 2p+q + 100 & = 100 - 5 = \boxed{95} \end{aligned}

Rifat Khan
Aug 24, 2013

x3+px2+qx+2 = (x-α)(x-β)(x-γ) so αβ=-1 ,or -p = α+β+γ , or q= αβ+βγ+αγ -αβγ=2 so γ=2 sub γ=2 into q= αβ+βγ+αγ and -p = α+β+γ q = -1+2(α+β) p = -2-(α+β) 2p+q+100 = 2(-2-(α+β))+-1+2(α+β)+100 = 95

Alan Liang
Aug 21, 2013

notice aby = - 2

thus y = 2, ab = -1

ab + by + ay = q

thus -1 + 2 (a+b) = q

a+ b = -(p+2)

thus -1 - 2(p+2) = q

thus 2p-1-2p - 4 + 100 = 95 the answer

Víctor Martín
Aug 21, 2013

We know that the binomial of grade 0, in this case "2" is equal the the product of the roots (I will call them a,b and c). a \times b = -1 and a·b·c=2; so c=-2 . Then we get x^{3}+p·x^{2}+q·x+2=x^{3} -2·x^{2} -x+2; so p=-2 and q=1; then 2·(-2) -1 +100= 95

Dan Bussell
Aug 20, 2013

Well we know that AB=-1 and we can use the fact that ABY=-d/a to show that ABY=-2 and so we can deduce that Y=2 so one root is x=2. After subbing in the value of 2 into the polynomial set equal to 0 we get 4p+2q=-10. Divide by 2 and get 2p+q=-5 then add 100 to get 2p+q+100=95

Aditya Karekatte
Aug 20, 2013

Since we know that (alpha)(beta) = -1, alpha = -1, beta = 1.

Substituting these in the given equations, we get p+q = 3 and p-q = -1. Therefore, p = -2 and q = -1. Then we can substitute that in the required equation.

Dhruv Sharma
Aug 19, 2013

Firstly we have that α β + 1 = 0 α β γ = γ \alpha \beta +1 = 0 \Rightarrow \alpha \beta \gamma = -\gamma

However, α β γ = 2 ) γ = 2 \alpha \beta \gamma = -2) \Rightarrow \gamma = 2

Next we have α β + β γ + γ α = q 1 + 2 β + 2 α = q \alpha \beta + \beta \gamma + \gamma \alpha = q \Rightarrow -1 + 2 \beta + 2 \alpha = q

And that α + β + γ = p \alpha + \beta + \gamma = p

Therefore 2 p + q + 100 = 2 ( α + β + γ ) + 2 β + 2 α 1 + 100 2p + q +100 = -2(\alpha + \beta + \gamma) + 2 \beta +2 \alpha -1 +100

2 ( α + β ) 4 + 2 ( α + β ) + 100 \Rightarrow -2(\alpha + \beta) - 4 +2 (\alpha + \beta) + 100

2 p + q + 100 = 95 \Rightarrow 2p+q+100 = 95

Ariba Siddiqui
Aug 19, 2013

f(x): x 3 x^{3} + p x 2 x^{2} + qx + 2 and αβ = -1.

From basic properties of cubic polynomial, product of roots αβγ= -2 ...putting αβ = -1 , we get γ = 2. Now since γ is a root of f(x), f(γ) = 0 {again through basic properties of polynomials and their roots} ...So, f(2) => 8 + p(4) + q(2) + 2 = 0. => 4p + 2q + 10 = 0. => 2p + q + 5 = 0. => 2p + q + (95+5) = 0+95. Hence the value 95. :)

Archit Goel
Aug 19, 2013

We know that the product of the roots of this equation must be -(2/1)

So , αβγ = -2

αβ+1=0

hence , γ = 2.. Now we know one root that is 2.

Substitute x=2 in the equation.

8+4p+2q+2=0

2p+q+5=0

2p+q+100=95

Daniel Cohen
Aug 18, 2013

Using AB + 1 = 0, we can conclude that A and B are opposites, so replace B with -A. So now the roots are A, -A, and C.

Using the factor theorem, we know the polynomial can be written as (x - A)(x + A)(x - C).

Expanding the polynomial, we get:

x^3 - (C)(x^2) - (A^2)(x)+ (A^2)(C) = x^3 + (p)(x^2) + (q)(x^3) + 2

It follows that:

i) p = -C ii) q = -(A^2)

and

iii) 2 = (A^2)(C)

It follows from iii) that C > 0 because (A^2) > 0.

I then tried to test only integer valued numbers for A and C to make (A^2)(C) = 2, and the only combination that works is A = 1 or A = -1, and C = 2. Then i) p = -2 and ii) q = -1.

2p + q + 100 = 2(-2) -1 + 100 = 95.

The only problem is that I don't know why the roots have to be integers. Do they? It seems that they do because the problem has a unique solution.

Ignore my solution. That's what I get for trying to do math at 2 am. Of course AB + 1 = 0 does not mean A & B are opposites. Negative reciprocals, yes.

Daniel Cohen - 7 years, 9 months ago
Jian Feng Gao
Aug 18, 2013

Let a, b, c be the roots of the polynomial.

From vieta's formulas we know that a+b+c=-p , ab+ac+bc=q , abc=-2 and also were given that ab=-1

ab+ac+bc=q (ab=-1)

ac+bc=q+1

c(a+b)=q+1 (c=-2/ab)

-2(a+b) / (ab) = q+1 (ab=-1)

2(a+b) = q+1 (a+b=-p-c)

-2p - 2c = q+1

-2c = 2p + q +1 (Add 99 to both sides)

99-2c = 2p + q + 1 (we know that c=-2ab and ab=-1 so c=2)

99-2(2) = 2p + q + 1

Therefore 2p + q + 1 = 95.

Daniel Wang
Aug 18, 2013

The fact that it doesn't specify what α \alpha and β \beta has to be gives us an incentive to slack off. The simplest case would be to assume that a=1, b=-1. By vieta's, γ = 2 \gamma = 2 , and expanding (x-1)(x+1)(x-2) gives p=-2, q=-1, giving answer of 95.

While you may have arrived the correct numerical answer, you should know that your solution is not valid. There are actually infinite polynomials that satisfy the given conditions. Testing the values from one of these polynomials doesn't give you the right to say 2 p + q + 100 2p+q+100 always equals 95 95 .

Mursalin Habib - 7 years, 9 months ago

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