Answer all the questions. (a) Express 62−92−1\dfrac{6\sqrt{2}-9}{\sqrt{2}-1} in the form of a+b2a+b\sqrt{2}, where aa and bb are integers. (b) 62−92−1\dfrac{6\sqrt{2}-9}{\sqrt{2}-1} is a root of the equation x2+px+q=0x^{2}+px+q=0 where pp and qq are integers. Using the result in part (a), find the value of pp and of qq.

  1. (a)

    Express 62−92−1\dfrac{6\sqrt{2}-9}{\sqrt{2}-1} in the form of a+b2a+b\sqrt{2} , where aa and bb are integers.

  2. (b)

    62−92−1\dfrac{6\sqrt{2}-9}{\sqrt{2}-1} is a root of the equation x2+px+q=0x^{2}+px+q=0 where pp and qq are integers.

    Using the result in part (a), find the value of pp and of qq.

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Worked solution

1(b)

1. Evaluate the expression from part (a) to find the root

62−92−1=(62−9)(2+1)(2−1)(2+1)=12+62−92−92−1=3−32\frac{6\sqrt{2}-9}{\sqrt{2}-1} = \frac{(6\sqrt{2}-9)(\sqrt{2}+1)}{(\sqrt{2}-1)(\sqrt{2}+1)} = \frac{12 + 6\sqrt{2} - 9\sqrt{2} - 9}{2 - 1} = 3 - 3\sqrt{2}

2. Substitute the root into the given quadratic equation

(3−32)2+p(3−32)+q=0(3 - 3\sqrt{2})^2 + p(3 - 3\sqrt{2}) + q = 0

3. Expand the equation and group rational and irrational terms

9−182+18+3p−3p2+q=09 - 18\sqrt{2} + 18 + 3p - 3p\sqrt{2} + q = 0
(27+3p+q)−(18+3p)2=0(27 + 3p + q) - (18 + 3p)\sqrt{2} = 0

4. Compare coefficients to find $p$ and $q$

Since pp and qq are integers, the rational and irrational parts must independently equal zero.
18+3p=0  ⟹  p=−618 + 3p = 0 \implies p = -6
27+3(−6)+q=0  ⟹  27−18+q=0  ⟹  q=−927 + 3(-6) + q = 0 \implies 27 - 18 + q = 0 \implies q = -9

Answer

p=−6p = -6, q=−9q = -9

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