Find the smallest positive integer e such that the polynomial a x 4 + b x 3 + c x 2 + d x + e has integer coefficients and has roots at − 2 , 3 , 5 / 3 , and − 1 / 2 .
Details and assumptions
A root of a polynomial is a number r where the polynomial is equal to 0. For example, the number r = 3 is a root of the polynomial 2 x − 6 .
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This problem can also be solved using vieta's formulas: From vietas'formulas we have :
a − b = 6 1 3 ( 1 )
a e = 5 ( 2 ) . From the second equation we obtain: e = 5 a .In order to have a minimum for e we must find the minimum of a .From the first equation we obtain :
a = 1 3 − 6 b .From this equation we can see in order to find the minimum of a we must first find the minimum of b .Since a is a positive integer b must be a negative integer. and 6 b must be divisible by 1 3 .Easily we find b = − 1 3 where we obtain a = 6 and e = 3 0 .I know this might look a little long but i had no idea of this Rational Root Theorem.
we can find smaller values for b but observe that if b is negative if b decreases , a increases which is something that we do not want.So in order to find the minimum of a we must find the maximum of b since b is negative.So sorry i expressed myself wrong.
I am impressed with the skill all of you have in solving these problems. I used the fact that the product of the roots will always equal the numerical term and then found the integer multiple of all the roots to get the answer -2x3x5/3x-1/2= 5 and 6x( 5) = 30 multiplying by 6 to get rid of the fractions
For the polynomial of degree 4, we are given the four roots.
( x + 2 ) ( x − 3 ) ( 2 x + 1 ) ( 3 x − 5 ) = a x 4 + b x 3 + c x 2 + d x + e
Since we only need the e term, we can multiply the second term in each two-termed part of the expression.
( 2 ) ( − 3 ) ( 1 ) ( − 5 ) = 3 0
This solution completely misses the aspect of "find the smallest positive integer e ".
For example, the polynomial 0 has roots at the 4 given points (and several others).
Let ax^4+bx^3+cx^2+dx+e = k(x+2)(x-3)(x-5÷3)(x+1/2) = k(x^4-13x^3÷3+47x÷6-17x^2÷3+5) For e to be minimum and the coefficients to be integer k =6. So e = 6×5 = 30.
I really like your solution. Try using LaTeX next time though so we can read it more easily.
Given the roots, we can conclude the following values of r (or x , since they're interchangeable):
x 1 a = − 2 ; x 2 = 3 ; x 3 = 3 5 ; x 4 = − 2 1
By multiplying the denominators and transposing the constants, we find that
a x 4 + b x 3 + c x 2 + d x + e = ( x + 2 ) ( x − 3 ) ( 3 x − 5 ) ( 2 x + 1 )
Since e can only be found by multiplying the constants:
e = ( 2 ) ( − 3 ) ( − 5 ) ( 1 ) = 3 0
If the polynomial a x 4 + b x 3 + c x 2 + d x + e has roots − 2 , 3 , 3 5 , − 2 1 then a x 4 + b x 3 + c x 2 + d x + e = a ( x + 2 ) ( x − 3 ) ( x − 3 5 ) ( x + 2 1 ) . Thus e is not necessarily equal to 30. Note that the problem ask for the smallest value of e .
*typo, I meant x 1 = − 2
the roots are given namely;
x = (-2), (3), ( 3 5 ), and (- 2 1 )
it can be written as
(x + 2)(x - 3)(3x - 5)(2x + 1) = 0
finding the "e", we can just multiply the terms without a variable:
(2)(-3)(-5)(1) = 30
Since the polynomial:
P ( x ) = a x 4 + b x 3 + c x 2 + d x + e
is required to have integer coefficients and the roots are indeed rational numbers, we may apply the rational roots theorem , which states that the any rational root β α is such that α divides e and β divides a . From here, we go through the roots and determine that 2 , 3 , and 5 must divide e . The lowest possible common factor is then, 3 0 , which is the solution.
Since a x 4 + b x 3 + c x 2 + d x + e has roots at − 2 , 3 , 3 5 , and − 2 1
We could say that ( x + 2 )( x − 3 )( 3 x − 5 )( 2 x + 1 ) = 0
Then,
( x 2 − x − 6 )( 6 x 2 − 7 x − 5 ) = 0
6 x 4 − 5 5 x 3 − 3 4 x 2 + 4 7 x + 3 0 = 0
Thus, e = 3 0
is the (3x-5) there because of factorization? I accidentally went (x-3/5)
We know that the roots of this can be expressed as:
* b/a = r1+r2+r3..r5 *
In order for this to have integer coefficients, *a must be a multiple of both 3 and 2, in order to cancel out 5/3 and -1/2 .*
We also know that e can be expressed as the multiple of all the roots of the polynomial.
Henceforth, it is easy to conclude that e/a = 5.
Since a is a multiple of 6 (in its lowest form), the answer is * *e=6 5 = 30.**
In order to solve this problem, you have to realize that the roots of the polynomial were found by setting each factor equal to 0. So if a root is -2, then it came from the factor (x + 2). By using this, the factors of the polynomial can be found to be (x + 2)(x - 3)(x - 5/3)(x + 1/2). By multiplying this out, you find that e equals 30.
In fact, e is divisible by 30, thus is not necessarily equal to 30.
Let the equation be f(x), it can be rewritten as such
(x+2)(x-3)(x-5/3)(x+1/2) = 0
This would give u all the roots in the question, however the polynomial has to have integer coefficients, expanding with a fraction inside would result in some coefficients to be a fraction.
Thus written as,
(x+2)(x-3)(3x-5)(2x-1)=0
As we are looking for the constant, there is no need to expand the whole equation, just take the constants in the equation and multiply them together.
So we have,
2 x -3 x -5 x 1= 30
Hope this helps!
Since -2 , 3 , 5/3 , and -1/2 are the roots of the equation, this implies that ( x + 2 ) ( x + 3 ) ( 3 x − 5 ) ( 2 x + 1 ) = 0 Expanding these brackets we get ( x 2 + 6 x 2 + 6 ) ( 6 x 2 − 7 x − 5 ) = 0 ( 6 x 4 + 2 9 x 3 − 1 1 x 2 − 7 2 x − 3 0 ) = 0 But a x 4 + b x 3 + c x 2 + d x + e = 6 x 4 + 2 9 x 3 − 1 1 x 2 − 7 2 x − 3 0
therefore the
smallest positive integer of e
equals
3
0
sorry guys (x-3) instead of (x+3) and -6 instead of +6 and +30 instead of -30..sorry for the mess..
A long method !
Let p,q,r,s be the roots of the equation $$ax^4 +bx^3 +cx^2 +dx +e$$ Where $$p=-2, q=3, r=\frac {5}{3}, s=\frac {-1}{2} $$ As we know, product of roots of equation= $$\frac {e}{a}$$ Hence, $$p * q * r * s=\frac {e}{a} $$ $$\Rightarrow \frac {e}{a}= \frac {30}{6} $$ Again, sum of roots=$$\frac {-b}{a}$$ $$\Rightarrow \frac {-b}{a}= p+q+r+s $$ $$\Rightarrow \frac {-b}{a}= \frac {13}{6}$$ Again, sum of product of two roots taken at a time=$$\frac {c}{a}$$ $$\Rightarrow p * q+q * r+r * s+s * p= \frac {c}{a} $$ $$\Rightarrow \frac {c}{a}= \frac {-5}{6}$$ Again, sum of product of three roots taken at a time= $$\frac {-d}{a}$$ $$\Rightarrow \frac {-d}{a}=p * q * r + q * r * s + r * s * p + s * p * q $$ $$\Rightarrow \frac {-d}{a}=-7$$ $$\Rightarrow \frac {-d}{a}=\frac {42}{6} $$ Hence we get; $$ a=6,b=-13,c=-5,d=42, \boxed {e=30}$$
sorry,i made a mistake in second last step where i wrote $$\frac {-d}{a}=\frac {42}{6}$$ Correct statement is; $$\frac {-d}{a}=\frac {-42}{6}$$
With Vietha's theorem that e/a = -2 . 3 . 5/3 . (-1/2)= 5, Because it consist 2,3, and 5 so, 5 x 3 x 2 = 30.
(x + 2)(x - 3)(3x - 5)(2x + 1) .... when we expand this, the constant term 'e' is the product of all constant terms in the brackets.
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By the Rational Root Theorem, we must have 2 ∣ e , 3 ∣ e , and 5 ∣ e . This means e ≥ 3 0 , achievable with
p ( x ) = ( x + 2 ) ( x − 3 ) ( 3 x − 5 ) ( 2 x + 1 )