Integer Coefficients

Algebra Level 3

Find the smallest positive integer e e such that the polynomial a x 4 + b x 3 + c x 2 + d x + e ax^4 + bx^3 + cx^2 +dx + e has integer coefficients and has roots at 2 -2 , 3 3 , 5 / 3 5/3 , and 1 / 2 -1/2 .

Details and assumptions

A root of a polynomial is a number r r where the polynomial is equal to 0. For example, the number r = 3 r = 3 is a root of the polynomial 2 x 6 2x - 6 .


The answer is 30.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

14 solutions

Akshaj Kadaveru
Dec 26, 2013

By the Rational Root Theorem, we must have 2 e 2|e , 3 e 3|e , and 5 e 5|e . This means e 30 e \ge 30 , achievable with

p ( x ) = ( x + 2 ) ( x 3 ) ( 3 x 5 ) ( 2 x + 1 ) p(x) = (x+2)(x-3)(3x-5)(2x+1)

This problem can also be solved using vieta's formulas: From vietas'formulas we have :

b a = 13 6 \frac{-b}{a}=\frac{13}{6} ( 1 ) (1)

e a = 5 \frac{e}{a}=5 ( 2 ) (2) . From the second equation we obtain: e = 5 a e=5a .In order to have a minimum for e e we must find the minimum of a a .From the first equation we obtain :

a = 6 b 13 a=\frac{-6b}{13} .From this equation we can see in order to find the minimum of a a we must first find the minimum of b b .Since a a is a positive integer b b must be a negative integer. and 6 b 6b must be divisible by 13 13 .Easily we find b = 13 b=-13 where we obtain a = 6 a=6 and e = 30 e=30 .I know this might look a little long but i had no idea of this Rational Root Theorem.

Lorenc Bushi - 7 years, 5 months ago

we can find smaller values for b b but observe that if b b is negative if b b decreases , a a increases which is something that we do not want.So in order to find the minimum of a a we must find the maximum of b b since b b is negative.So sorry i expressed myself wrong.

Lorenc Bushi - 7 years, 5 months ago

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

Patrick O'Connell - 7 years, 5 months ago
David Caso
Aug 4, 2013

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 (x+2)(x-3)(2x+1)(3x-5) = ax^{4}+bx^{3}+cx^2+dx+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 ) = 30 (2)(-3)(1)(-5) = 30

Moderator note:

This solution completely misses the aspect of "find the smallest positive integer e e ".

For example, the polynomial 0 0 has roots at the 4 given points (and several others).

Aryan C.
Dec 27, 2013

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.

Carl Denton - 7 years, 5 months ago
Zechariah Jimenez
Dec 26, 2013

Given the roots, we can conclude the following values of r r (or x x , since they're interchangeable):

x 1 a = 2 ; x 2 = 3 ; x 3 = 5 3 ; x 4 = 1 2 \displaystyle x_1a=-2;x_2=3;x_3=\frac{5}{3};x_4=-\frac{1}{2}

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 ) \displaystyle ax^4+bx^3+cx^2+dx+e=(x+2)(x-3)(3x-5)(2x+1)

Since e e can only be found by multiplying the constants:

e = ( 2 ) ( 3 ) ( 5 ) ( 1 ) = 30 e=(2)(-3)(-5)(1)=\boxed{30}

If the polynomial a x 4 + b x 3 + c x 2 + d x + e ax^4+bx^3+cx^2+dx+e has roots 2 , 3 , 5 3 , 1 2 -2, 3, \frac{5}{3}, -\frac{1}{2} then a x 4 + b x 3 + c x 2 + d x + e = a ( x + 2 ) ( x 3 ) ( x 5 3 ) ( x + 1 2 ) ax^4+bx^3+cx^2+dx+e=a(x+2)(x-3)(x-\frac{5}{3})(x+\frac{1}{2}) . Thus e e is not necessarily equal to 30. Note that the problem ask for the smallest value of e e .

Jorge Tipe - 7 years, 5 months ago

*typo, I meant x 1 = 2 x_1=-2

Zechariah Jimenez - 7 years, 5 months ago
Francis Naldo
Aug 5, 2013

the roots are given namely;

x = (-2), (3), ( 5 3 \frac{5}{3} ), and (- 1 2 \frac{1}{2} )

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

Kevin Multani
Aug 8, 2013

Since the polynomial:

P ( x ) = a x 4 + b x 3 + c x 2 + d x + e P(x)= ax^4+bx^3+cx^2+dx+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 α β \frac{\alpha}{\beta} is such that α \alpha divides e e and β \beta divides a a . From here, we go through the roots and determine that 2 2 , 3 3 , and 5 5 must divide e e . The lowest possible common factor is then, 30 30 , which is the solution.

Since a x 4 ax^{4} + b x 3 bx^{3} + c x 2 cx^{2} + d x dx + e e has roots at 2 , 3 , 5 3 , -2, 3, \frac {5}{3}, and 1 2 -\frac {1}{2}

We could say that ( x + 2 x+2 )( x 3 x-3 )( 3 x 5 3x-5 )( 2 x + 1 2x+1 ) = 0 =0

Then,

( x 2 x 6 x^{2}-x-6 )( 6 x 2 7 x 5 6x^{2}-7x-5 ) = 0 =0

6 x 4 55 x 3 34 x 2 + 47 x + 30 6x^{4}-55x^{3}-34x^{2}+47x+30 = 0 =0

Thus, e = 30 e=30

is the (3x-5) there because of factorization? I accidentally went (x-3/5)

David Kroell - 7 years, 10 months ago
Anirudh Gopinath
Dec 27, 2013
  • 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.**

Hunter Smutney
Dec 26, 2013

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 e is divisible by 30, thus is not necessarily equal to 30.

Jorge Tipe - 7 years, 5 months ago
The Math Slayer
Jan 6, 2014

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!

Abubakarr Yillah
Jan 3, 2014

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 ({x}+{2})({x}+{3})({3x}-{5})({2x}+{1})={0} Expanding these brackets we get ( x 2 + 6 x 2 + 6 ) ( 6 x 2 7 x 5 ) = 0 ({x^2}+{6x^2}+{6})({6x^2}-{7x}-{5})={0} ( 6 x 4 + 29 x 3 11 x 2 72 x 30 ) = 0 ({6x^4}+{29x^3}-{11x^2}-{72x}-{30})={0} But a x 4 + b x 3 + c x 2 + d x + e = 6 x 4 + 29 x 3 11 x 2 72 x 30 {ax^4}+{bx^3}+{cx^2}+{dx}+{e}= {6x^4}+{29x^3}-{11x^2}-{72x}-{30}

therefore the smallest positive integer of e equals
30 \boxed{30}

sorry guys (x-3) instead of (x+3) and -6 instead of +6 and +30 instead of -30..sorry for the mess..

Abubakarr Yillah - 7 years, 5 months ago

A long method !

Niranjan Khanderia - 7 years, 3 months ago
Caroline Sudipa
Dec 30, 2013

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; $$\frac {-d}{a}=\frac {-42}{6}$$

Caroline Sudipa - 7 years, 5 months ago
Budi Utomo
Dec 28, 2013

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.

Bharat Karmarkar
Dec 27, 2013

(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.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...