[微分] 89清大經濟

[monene5566 (機掰凱文)]

         2        2
Let f:| R  --> | R  be differentiable . Given u=(3/5,4/5), v=(4/5,-3/5)

                        2      '  2      '  2
prove that | gradient f |  = |f_u|  +  |f_v|

       '       '
where f_u and f_v are the directional derivative  of f in the direction

of u and v

這題不知道該怎樣證  連第一步要怎樣進行都無頭緒

請版友解題了  謝謝:)

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