If the roots of the equation x 4 − a x 2 + b = 0 are four consecutive odd integers, what is the value of a + b ?
This problem is posed by Mohnish C .
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you can also use the vieta formulae.
according to vieta souldnt
-a = sum of product of roots taken two at a time?
i.e a = -(3-1+3-9) = 4
can you tell me bout
the viete relationn
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Yes of course!!! here it is - http://mathworld.wolfram.com/VietasFormulas.html
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or you may visit - http://en.wikipedia.org/wiki/Vieta's_formulas
If r is a solution, so is − r . Thus, the four consecutive integers are − 3 , − 1 , 1 , 3 . Plugging in x = 3 , 1 , we obtain b = 9 a − 8 1 b = a − 1 Thus, a − 1 = 9 a − 8 1 ⟹ a = 1 0 ⟹ b = 9 . Indeed, the equation x 4 − 1 0 x 2 + 9 has roots at x = ± 1 , 3 , and our answer is 1 0 + 9 = 0 1 9 .
Because r 2 = ( − r ) 2 and r 4 = ( − r ) 4 because of the even powers.
Can you connect your argument? Why do we have "If r is a solution, so is − r " ?
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 − 1 0 x 2 + 9 so a + b = 1 9 .
Let the roots of the equation be w − 2 , w , w + 2 and w + 4 with w being odd. Using Vieta's formula with respect to the coefficient on the x 3 term, which is zero, we get:
0 = w − 2 + w + w + 2 + w + 4 , meaning that w = − 1 . Thus our four roots are -3, -1, 1 and 3. Thus our polynomial is ( x + 3 ) ( x + 1 ) ( x − 1 ) ( x − 3 ) , which when expanded and simplified, leaves us with x 4 − 1 0 x 2 + 9 . Therefore our answer is 1 0 + 9 = 1 9
why root cannot be w,w+2,w+4,w+6?
Let the 4 roots be α − 2 , α , α + 2 , α + 4 .
By Vieta's Formula,
α − 2 + α + α + 2 + α + 4 = 0
α = − 1
Hence, the roots are − 3 , − 1 , 1 , 3 .
Substitute the root 1 and 3 into the equation,
1 − a + b = 0
8 1 − 9 a + b = 0
Solve the system of equation,
a = 1 0 and b = 9
Hence, a + b = 1 9 .
why root cannot be w,w+2,w+4,w+6
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Yes you can, only you will find
w + w + 2 + w + 4 + w + 6 = 0
w = − 3
Hence, the four roots are
− 3 , − 1 , 1 , 3 .
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.
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.
As co-efficient of 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
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
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
If we think that S = ( 1 , 3 , 5 , 7 )
We have:
x 4 − a x 2 + b = 0 = > 1 4 − a × 1 2 + b = 0 = >
1 − a + b = 0 = > a = b + 1
Nice... we have an equation:
a = b + 1
The next value of x
x 4 − a x 2 + b = 0 replece the x by 3
3 4 − a × 3 2 + b = 0
8 1 − 9 a + b = 0 = > 9 a = 8 1 + b
replace a
9 ( b + 1 ) = 8 1 + b = > 9 b + 9 = 8 1 + b
9 b − b = 8 1 − 9 = > 8 b = 7 2 = > b = 8 7 2 = > b = 9
Replace b in first equation:
a = b + 1 = > a = 9 + 1 = > a = 1 1
So:
a + b = 1 0 + 9 = > a + b = 1 9
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 and 7 into x 4 − 1 0 x 2 + 9 .
Can you explain what you mean by "If we think that S = ( 1 , 3 , 5 , 7 ) "?
Sure! The S = ( 1 , 3 , 5 , 7 ) are the values of x that I have termed as a " S o l u t i o n "
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Since the roots of the given polynomial are four consecutive odd integers , then let the roots be- 2 k + 1 , 2 k + 3 , 2 k + 5 and 2 k + 7
By Viete's relation, we have -
( 2 k + 1 ) + ( 2 k + 3 ) + ( 2 k + 5 ) + ( 2 k + 7 ) = 0 ⇒ 8 k + 1 6 = 0 ⇒ k = − 2
Thus we get the roots of the polynomial as - − 3 , − 1 , 1 , 3
Now, substituting 1 (or − 1 ) in the given equation gives us -
1 − a + b = 0
Substituting 3 (or − 3 ) in the given equation gives us -
8 1 − 9 a + b = 0
Solving the above two equations, gives us -
a = 1 0 and b = 9
Thus a + b = 1 9