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# Rational Root Theorem Help

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Let f(x) be a polynomial with integer coefficients such that f(5) = f(7) = 20. What is the smallest possible value of |f(0)|?

Jan 5, 2020

#1
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A bit of a trick question!  If f(x) is the constant polynomial 20, then you get |f(0)| = 20, which is the minimum.

Jan 6, 2020
#2
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It says that it is still wrong.

Jan 8, 2020
#3
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If it did not have integer coefficients the answer would be 0.

BUT

with integer coefficients I do not know ???

If I had time and inclination I would go watch some videos on Rational Root Theorum and maybe I could figure it out.

BUT

I'd really like someone to show my how to do it here.

Any takers?

Jan 8, 2020
#4
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The minimum is, I think, 15.

Suppose that $$f(x)=a+bx+cx^{2}+...$$

then $$|f(0)|=a.$$

Substituting x=5 and x=7 gets you

$$20-a=5b+25c+...$$

and

$$20-a=7b+49c+...$$

For integer a, b and c, it's necessary that 20 - a should be divisible by both 5 and by 7.

The smallest, in magnitude, a, is -15.

The quadratic $$f(x)= -15+12x-x^{2}$$

works, I haven't looked for a cubic.

Jan 8, 2020
#5
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Thanks Guest.

I am still a big confused.

You have made some little error statements like it is   f(0)=a    without the absolute signs.

BUT

I think my main problem is that i do not understand root theory well. So I probably need to do some study.

Melody  Jan 9, 2020
#6
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Let f(x) be a polynomial with integer coefficients such that $$f(5) = f(7) = 20$$.
What is the smallest possible value of $$|f(0)|$$?

a)

$$f(x) = 20 + (5-x)(7-x) \qquad \Big(~ f(x)=x^2-12x+55 ~\Big)$$

$$\begin{array}{|rcll|} \hline f(0) &=& 20 + (5-0)(7-0) \\ f(0) &=& 20 + 35 \\ f(0) &=& 55 \\ \mathbf{|f(0)|} &=& \mathbf{55} \\ \hline \end{array}$$

b)
$$f(x) = 20 - (5-x)(7-x) \qquad \Big(~ f(x)=-x^2+12x-15 ~\Big)$$

$$\begin{array}{|rcll|} \hline f(0) &=& 20 - (5-0)(7-0) \\ f(0) &=& 20 - 35 \\ f(0) &=& -15 \\ \mathbf{|f(0)|} &=& \mathbf{15} \\ \hline \end{array}$$

The smallest possible value of $$|f(0)|$$ is 15

Jan 9, 2020
#7
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Thanks Heureka.

I will have to think about this one before I will be able to claim full understand.

Melody  Jan 9, 2020