$\displaystyle

\textup{min} f{}'(x)\textup{ }d $

$\displaystyle s.t \left \| d \right \|_{2}^{2} = 1 $

$\displaystyle \textup{show }d = -\frac{f'(x)}{\left \| f(x) \right \|_{2}} $

$\displaystyle \textup{my attempt} $

$\displaystyle \textup{Let } d = (d_{1}, ... ,d_{n}) $$\displaystyle

\newline \textup{constraint = } \sum d^{2}_{i} $$\displaystyle

\newline \textup{equation = } \sum f'(x)^{2} d^{2}_{i} $$\displaystyle

\newline \textup{I tryed using lagrangad: using } $$\displaystyle

L(d, \lambda) = \sum f'(x)^{2} d^{2}_{i}} - \lambda ( \sum d^{2}_{i} - 1) $but got the wrong anwser