Suppose α , β , and γ are the roots of polynomial x 3 + p x 2 + q x + 2 = 0 and that α β + 1 = 0 . What is the value of 2 p + q + 1 0 0 ?
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Nicely done!
Nicely done! I'm voting you up!
Very cool, I didn't think of using f ( 2 ) = 0
Thanks guys! :D
This is how I solved it :)
Solved by same method !
By Vieta's Formulas, the product of the roots is α β γ = − 2 . We know that α β = − 1 , so γ = 2 . Since 2 is a root of the polynomial, we have 2 3 + p ( 2 2 ) + q ( 2 ) + 2 = 0 , or 4 p + 2 q = − 1 0 . Dividing through by 2 gives 2 p + q = − 5 . Thus, 2 p + q + 1 0 0 = 9 5 .
Nicely done!
It seems like most solutions followed the same pattern: Vieta's theorem, then plugging in 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?
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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 ) and expand; then you can compare coefficients.
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.
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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.
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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.
Cool.
Hey Michael, do you play ftw? If you do, what is your rating?
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@Revanth Gumpu – It's around the 1500s. I don't play much anymore though.
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@Michael Tang – What is your username? Mine is kevjumba
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@Revanth Gumpu – I'll send you a PM on AoPS.
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@Michael Tang – kk
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@Revanth Gumpu – Dude, I know MSTang, wow, i didnt know that was you.
By Vieta's Formulas, p = α + β + γ , q = α β + α γ + β γ , and 2 = α β γ .
Since α β + 1 = 0 , or α β = − 1 , we have 2 = ( − 1 ) γ , so γ = − 2 .
From this, we have that p = α + β − 2 and q = α β − 2 ( α + β ) , so 2 p + q + 1 0 0 = 2 ( α + β − 2 ) + α β − 2 ( α + β ) + 1 0 0 .
Simplifying and plugging in our value for α β , we get our answer to be 2 ( − 2 ) − 1 + 1 0 0 = 9 5 .
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 :)
By using Vieta's Formula,we know that
α + β + γ = − p
α β + α γ + β γ = q
α β γ = − 2
In addition,we know that α β = − 1 ⇒ γ = 2
By substituting the value,we get
α + β = − p − 2 ⇒ 2 α + 2 β = − 2 p − 4
and
2 α + 2 β = q + 1
By equating the two equations,
− 2 p − 4 = q + 1 ⇒ 2 p + q = − 5 ⇒ 2 p + q + 1 0 0 = 9 5
Expanding and factoring gives:
p = − γ − β − α
q = β γ + α γ + α β
− α β γ = 2
Given α β + 1 = 0 then α β = − 1 and γ = 2
2 p + q + 1 0 0
= 2 ( − 2 − β − α ) + 2 β + 2 α + α β + 1 0 0
= α β + 9 6 = 9 5
Given that α β + 1 = 0 or α β = − 1 and using Vieta's formula, we have:
⎩ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎧ α β = − 1 α + β + γ = − p α β + β γ + γ α = q α β γ = − 2 . . . ( 0 ) . . . ( 1 ) . . . ( 2 ) . . . ( 3 )
( 0 ) → ( 3 ) : ( 1 ) : ( 2 ) : α β γ = ( − 1 ) γ = − 2 α + β + γ = α + β + 2 = − p α β + β γ + γ α = − 1 + 2 ( α + β ) = − 1 + 2 ( − p − 2 ) = q ⇒ γ = 2 ⇒ α + β = − p − 2 ⇒ q = − 5 − 2 p
q ⇒ 2 p + q 2 p + q + 1 0 0 = − 5 − 2 p = − 5 = 1 0 0 − 5 = 9 5
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
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
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
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
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.
Firstly we have that α β + 1 = 0 ⇒ α β γ = − γ
However, α β γ = − 2 ) ⇒ γ = 2
Next we have α β + β γ + γ α = q ⇒ − 1 + 2 β + 2 α = q
And that α + β + γ = p
Therefore 2 p + q + 1 0 0 = − 2 ( α + β + γ ) + 2 β + 2 α − 1 + 1 0 0
⇒ − 2 ( α + β ) − 4 + 2 ( α + β ) + 1 0 0
⇒ 2 p + q + 1 0 0 = 9 5
f(x): x 3 + p 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. :)
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
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.
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.
The fact that it doesn't specify what α and β 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 , 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 + 1 0 0 always equals 9 5 .
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From Vieta's Theorem, or by expanding ( x − α ) ( x − β ) ( x − γ ) = x 3 + p x 2 + q x + 2 , we get − α β γ = 2 .
α β + 1 = 0 ⇒ α β = − 1 , we find γ = 2 by substituting in the above equation.
γ = 2 is a root, so
2 3 + 2 2 p + 2 q + 2 = 0
4 p + 2 q + 1 0 = 0
2 p + q + 5 = 0
2 p + q + 1 0 0 = 9 5