It is given that aa and bb are the roots of the quadratic equation x2−2x−1=0x^2 - 2x - 1 = 0 and that a>ba > b. Show that ab=−3−22\frac{a}{b} = -3 - 2\sqrt{2}.

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Worked solution

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1. Find the roots of the quadratic equation using the quadratic formula

x=−(−2)±(−2)2−4(1)(−1)2(1)x = \frac{-(-2) \pm \sqrt{(-2)^2 - 4(1)(-1)}}{2(1)}
x=2±4+42x = \frac{2 \pm \sqrt{4 + 4}}{2}
x=2±82x = \frac{2 \pm \sqrt{8}}{2}
x=2±222x = \frac{2 \pm 2\sqrt{2}}{2}
x=1±2x = 1 \pm \sqrt{2}

2. Identify $a$ and $b$ using the given condition $a > b$

Since a>ba > b, we have:
a=1+2a = 1 + \sqrt{2}
b=1−2b = 1 - \sqrt{2}

3. Set up the fraction $\frac{a}{b}$ and rationalise the denominator

ab=1+21−2\frac{a}{b} = \frac{1 + \sqrt{2}}{1 - \sqrt{2}}
Multiply numerator and denominator by the conjugate of the denominator, (1+2)(1 + \sqrt{2}):
=(1+2)(1+2)(1−2)(1+2)= \frac{(1 + \sqrt{2})(1 + \sqrt{2})}{(1 - \sqrt{2})(1 + \sqrt{2})}

4. Expand and simplify to show the final result

=1+22+(2)212−(2)2= \frac{1 + 2\sqrt{2} + (\sqrt{2})^2}{1^2 - (\sqrt{2})^2}
=1+22+21−2= \frac{1 + 2\sqrt{2} + 2}{1 - 2}
=3+22−1= \frac{3 + 2\sqrt{2}}{-1}
=−3−22 (shown)= -3 - 2\sqrt{2} \text{ (shown)}

Answer

\frac{a}{b} = -3 - 2\sqrt{2}

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