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I understand everything until the last part where they get $\frac{2}{5} \sqrt{5}$?

I know the $\lVert [2\ -4] \rVert$ means $\sqrt{2^2 + -4^2}$

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1 Answer

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Yes it is right, and also you, we have in fact $|| (2,-4)||= \sqrt{2^2+(-4)^2}= \sqrt{20}=\sqrt{4•5}=2\sqrt{5}$

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