a/ $M=(\sqrt 8 -4\sqrt 2 +\sqrt{40}).\sqrt 2\\=\sqrt{8}.\sqrt{2}-4\sqrt{2}.\sqrt{2}+\sqrt{40}.\sqrt{2}\\=\sqrt{16}-4\sqrt{4}+\sqrt{80}\\=4-4.2+2\sqrt{10}\\=4-8+2\sqrt{10}\\=-4+2\sqrt{10}$
$N=\dfrac{1}{\sqrt 5+2}\\=\dfrac{\sqrt 5-2}{(\sqrt 5+2)(\sqrt 5-2)}\\=\dfrac{\sqrt 5-2}{(\sqrt 5)^2-2^2}\\=\dfrac{\sqrt 5-2}{5-4}\\=\dfrac{\sqrt 5-2}{1}\\=\sqrt 5-2$
