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4.5 Directional derivatives

1.

(a) $\displaystyle{-\sqrt{2}}$ (b) $\displaystyle{-\frac{e}{\sqrt{2}}}$

2.

(a) $\displaystyle{\sqrt{3}}$ (b) $\displaystyle{1}$　

3.

(a) $\displaystyle{\frac{3}{\sqrt{10}}, (0,1)}$(b) $\displaystyle{-\frac{6}{\sqrt{10}}, (0,-1)}$