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# The answer is not 38/53

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Let p(x) be a quadratic polynomial with integer coefficients which has $$4-\sqrt{11}$$ as a root. Compute $$\frac{p(3)}{p(4)}$$.

May 28, 2019

#1
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Hi Lightning

$$P(x)=ax^2+bx+c\\ P(4-\sqrt{11})=a(4-\sqrt{11})^2+b(4-\sqrt{11})+c=0\\\qquad \text{Where a,b and c are integers}\\ a(4-\sqrt{11})^2+b(4-\sqrt{11})+c=0\\ a(16+11-8\sqrt{11})+b(4-\sqrt{11})+c=0\\ a(27-8\sqrt{11})+b(4-\sqrt{11})+c=0\\ 27a+4b=-c\quad(1)\\ -8a\sqrt{11}-b\sqrt{11}=0\\ -8a-b=0\\ b=-8a \quad (2)\\ \text{sub two into one}\\ 27a+4*-8a=-c\\ -5a=-c\\ c=5a$$

So the equation becomes

$$P(x)=ax^2-8ax+5a\\ P(x)=a(x^2-8x+5)\\$$

Find P(3) and P(4) and do the division.

You can finish it

(oh, you do need to check my working, I have not done so. )

May 28, 2019
#2
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I'd make this a bit simpler.

$$\text{We can always choose P(x) to be a monic polynomial}\\ \text{Integer coefficients means 4-\sqrt{11} is a root \Rightarrow 4+\sqrt{11} is a root}\\ \text{Thus we have}\\ P(x) = \left(x- 4+\sqrt{11}\right)\left(x-4-\sqrt{11}\right)=x^2-8x+5$$

Rom  May 28, 2019
edited by Rom  May 28, 2019
#3
+106921
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That is much nicer Rom :)

Melody  May 28, 2019