• Oct 29th 2009, 04:27 PM
JohnLeee
http://i37.tinypic.com/2nb7n5.jpg

How do I approach this question?
Thanks
• Oct 29th 2009, 06:23 PM
Jose27
Let $\nabla f(a)=v$ then $u\cdot v= \Vert u \Vert \Vert v \Vert \cos \theta$ and since $\Vert u \Vert =1$ we get that $\vert D_uf \vert$ reaches a maximum when $\cos \theta = \pm 1$ which happens iff $u = \lambda v$ for some $\lambda \in \mathbb{R}$