Mohnish's polynomial

Algebra Level 4

If the roots of the equation x 4 a x 2 + b = 0 x^4 - ax^2 + b = 0 are four consecutive odd integers, what is the value of a + b a+b ?

This problem is posed by Mohnish C .


The answer is 19.

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13 solutions

Kishlaya Jaiswal
Nov 13, 2013

Since the roots of the given polynomial are four consecutive odd integers , then let the roots be- 2 k + 1 2k+1 , 2 k + 3 2k+3 , 2 k + 5 2k+5 and 2 k + 7 2k+7

By Viete's relation, we have -

( 2 k + 1 ) + ( 2 k + 3 ) + ( 2 k + 5 ) + ( 2 k + 7 ) = 0 8 k + 16 = 0 k = 2 (2k+1)+(2k+3)+(2k+5)+(2k+7) = 0 \Rightarrow 8k+16 = 0 \Rightarrow k = -2

Thus we get the roots of the polynomial as - 3 , 1 , 1 , 3 -3,-1,1,3

Now, substituting 1 1 (or 1 -1 ) in the given equation gives us -

1 a + b = 0 1-a+b=0

Substituting 3 3 (or 3 -3 ) in the given equation gives us -

81 9 a + b = 0 81-9a+b=0

Solving the above two equations, gives us -

a = 10 a = 10 and b = 9 b = 9

Thus a + b = 19 a+b=\boxed{19}

you can also use the vieta formulae.

Krishna Ramesh - 7 years, 1 month ago

according to vieta souldnt

-a = sum of product of roots taken two at a time?

i.e a = -(3-1+3-9) = 4

Sahil Gohan - 7 years ago

can you tell me bout
the viete relationn

yee cheng - 7 years, 6 months ago

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Yes of course!!! here it is - http://mathworld.wolfram.com/VietasFormulas.html

Kishlaya Jaiswal - 7 years, 6 months ago

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or you may visit - http://en.wikipedia.org/wiki/Vieta's_formulas

Kishlaya Jaiswal - 7 years, 6 months ago
Nathan Weckwerth
Nov 11, 2013

If r r is a solution, so is r -r . Thus, the four consecutive integers are 3 , 1 , 1 , 3 -3,-1,1,3 . Plugging in x = 3 , 1 x=3,1 , we obtain b = 9 a 81 b=9a-81 b = a 1 b=a-1 Thus, a 1 = 9 a 81 a = 10 b = 9 a-1=9a-81\implies a=10\implies b=9 . Indeed, the equation x 4 10 x 2 + 9 x^4-10x^2+9 has roots at x = ± 1 , 3 x=\pm 1,3 , and our answer is 10 + 9 = 019 10+9=\boxed{019} .

Because r 2 = ( r ) 2 r^2=(-r)^2 and r 4 = ( r ) 4 r^4=(-r)^4 because of the even powers.

Nathan Weckwerth - 7 years, 7 months ago

Can you connect your argument? Why do we have "If r r is a solution, so is r -r " ?

Calvin Lin Staff - 7 years, 7 months ago
Daniel Chiu
Nov 10, 2013

By Vieta's formula's, the sum of the roots is 0. If the roots are consecutive odd numbers that add to 0, then they are -3,-1,1,3. Finally, ( x 3 ) ( x 1 ) ( x + 1 ) ( x + 3 ) = x 4 10 x 2 + 9 (x-3)(x-1)(x+1)(x+3)=x^4-10x^2+9 so a + b = 19 a+b=\boxed{19} .

James Remo
Jan 25, 2014

Let the roots of the equation be w 2 w-2 , w w , w + 2 w+2 and w + 4 w+4 with w w being odd. Using Vieta's formula with respect to the coefficient on the x 3 x^3 term, which is zero, we get:

0 = w 2 + w + w + 2 + w + 4 0 = w-2+w+w+2+w+4 , meaning that w = 1 w=-1 . Thus our four roots are -3, -1, 1 and 3. Thus our polynomial is ( x + 3 ) ( x + 1 ) ( x 1 ) ( x 3 ) (x+3)(x+1)(x-1)(x-3) , which when expanded and simplified, leaves us with x 4 10 x 2 + 9 x^4 -10x^2 +9 . Therefore our answer is 10 + 9 = 19 10+9=\boxed{19}

why root cannot be w,w+2,w+4,w+6?

Yuan yuan - 7 years, 3 months ago

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you can go thru my solution

Arijit Banerjee - 7 years, 2 months ago
Christopher Boo
Mar 9, 2014

Let the 4 roots be α 2 , α , α + 2 , α + 4 \alpha-2, \alpha, \alpha+2, \alpha+4 .

By Vieta's Formula,

α 2 + α + α + 2 + α + 4 = 0 \alpha-2+\alpha+\alpha+2+\alpha+4=0

α = 1 \alpha=-1

Hence, the roots are 3 , 1 , 1 , 3 -3,-1,1,3 .

Substitute the root 1 1 and 3 3 into the equation,

1 a + b = 0 1-a+b=0

81 9 a + b = 0 81-9a+b=0

Solve the system of equation,

a = 10 a=10 and b = 9 b=9

Hence, a + b = 19 a+b=19 .

why root cannot be w,w+2,w+4,w+6

Yuan yuan - 7 years, 3 months ago

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Yes you can, only you will find

w + w + 2 + w + 4 + w + 6 = 0 w+w+2+w+4+w+6=0

w = 3 w=-3

Hence, the four roots are

3 , 1 , 1 , 3 -3,-1,1,3 .

Christopher Boo - 7 years, 2 months ago
Noel Lo
Nov 11, 2013

Let the four roots be (n-3), (n-1), (n+1) and (n+3). So we have (x-n-3)(x-n-1)(x-n+1)(x-n+3) = 0 Upon expansion, we get (x^2 - 2nx - (n^2-1))(x^2 - 2nx - (n^2-9))=0 Since the coefficients of x and x^3 are zero, 2n = 0 hence n=0. Now, a= -(n^2-1)-(n^2-9) = 10 - 2n^2 = 10. b= (n^2-1)(n^2-9) = (-1)(-9) = 9. Hence a+b = 19.

Arijit Banerjee
Apr 2, 2014

there is a rule( I forgot the name sorry ) check the number of changes in sign i.e. + to - then again from - to + ...(sign behind each terms) .. we can see two such changes so number of positive real roots will be two so the other two must me negative integers. since they are consecutive integers so the numbers are -3 , -1 , 1 , 3 put any two diff values and solve the two equations... u will get a + b = 19

Let the roots be 2n-3, 2n-1, 2n+1, 2n+3 where n is an integer since they are odd. Hence b = multiplication of all four, which is 16n^4- 40n^2 + 9. But n is an integer.
If we try n=0 we get b=9 and roots are -3,-1,3,1
putting x = -3, -1, 1, 3 and solving we always get b = 9 and a = 10. Thus n = 0 was correct try.
Note - I have taken the roots as I did is since it is the most convenient way to take odd numbers.


Sorry. b is x^2 term with lead cof. =1. Thus by adding root two at a time a=10.

Niranjan Khanderia - 7 years, 2 months ago

As co-efficient of x 3 x^{3} is 0, so it's clear that the four roots are -3, -1, 1, 3. Then just apply Vieta's formula to get the Value of a, b.

here a = 10, b = 9, a + b = 19

Just take an example of the roots being -3,-1,1,3 as it is applicable for any 4 consecutive odd integers with equation of the form x^4-a.x^2+b=0 .

The equation with the above roots are x^4-10.x^2+9=0 ====> a+b=19

by vietas theoram, sum of roots=0; also roots are 4 consecutive odd integers therefore the roots are -3,-1,1,3 implies the polynomial is (x-3)(x-1)(x+1)(x+3) therefore a=10&b=9

Sanjay Banerji
Nov 14, 2013

So simple...

Taking the roots as 2N-3 ; 2N-1 ; 2N+1 ; 2N+3

coefficient of x^3=0 =summation of roots = -8N

Since coefficient of x^3=0 thus we get n=0; the roots are -3,-1,+1,+3

By calculating; we get b=9 and a=10

Eduardo Teruo
Nov 11, 2013

If we think that S = ( 1 , 3 , 5 , 7 ) S = (1, 3, 5, 7)

We have:

x 4 a x 2 + b = 0 = > 1 4 a × 1 2 + b = 0 = > x^{4} - ax^{2} + b = 0 => 1^{4} - a\times1^{2} + b = 0 =>

1 a + b = 0 = > a = b + 1 1 - a + b = 0 => a = b + 1

Nice... we have an equation:

a = b + 1 a = b + 1

The next value of x x

x 4 a x 2 + b = 0 x^{4} - ax^{2} + b = 0 replece the x x by 3 3

3 4 a × 3 2 + b = 0 3^{4} - a\times3^{2} + b = 0

81 9 a + b = 0 = > 9 a = 81 + b 81 - 9a + b = 0 => 9a = 81 + b

replace a a

9 ( b + 1 ) = 81 + b = > 9 b + 9 = 81 + b 9 (b + 1) = 81 + b => 9b + 9 = 81 + b

9 b b = 81 9 = > 8 b = 72 = > b = 72 8 = > b = 9 9b - b = 81 - 9 => 8b = 72 => b = \frac{72}{8} => b = 9

Replace b b in first equation:

a = b + 1 = > a = 9 + 1 = > a = 11 a = b + 1 => a = 9 + 1 => a = 11

So:

a + b = 10 + 9 = > a + b = 19 a + b = 10 + 9 => \boxed{a + b = 19}

But that polynomial, when expanded, is not in the correct form. I think that you got lucky because the two numbers you tried are solutions of the equation. Try plugging in 5 5 and 7 7 into x 4 10 x 2 + 9 x^4-10x^2+9 .

Nathan Weckwerth - 7 years, 7 months ago

Can you explain what you mean by "If we think that S = ( 1 , 3 , 5 , 7 ) S = (1, 3, 5, 7) "?

Calvin Lin Staff - 7 years, 7 months ago

Sure! The S = ( 1 , 3 , 5 , 7 ) S = (1, 3, 5, 7) are the values of x x that I have termed as a " S o l u t i o n Solution "

Eduardo Teruo - 7 years, 7 months ago

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