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6.
Rationalise denominator
$\dfrac{1}{\sqrt{a}+\sqrt{b}}=\dfrac{\sqrt{a}-\sqrt{b}}{a-b}$
7.
Rationalise denominator
$\dfrac{1}{\sqrt{a}-\sqrt{b}}=\dfrac{\sqrt{a}+\sqrt{b}}{a+b}$
8.
Lagrange's Identity
$\sqrt{a \pm \sqrt{b}}=\dfrac{\sqrt{a+\sqrt{a^2-b}}}{2} \pm \dfrac{\sqrt{a-\sqrt{a^2-b}}}{2}$