For Function $f ( x , y ) = 8 - \frac { x } { 8 } - \frac { y } { 7 } , \text { find the maximum value of the directional }$

Multiple Choice

For function $f ( x , y ) = 8 - \frac { x } { 8 } - \frac { y } { 7 } , \text { find the maximum value of the directional }$ derivative at (3,2) .

A) $\sqrt { 15 } / 8$
B) $\sqrt { 113 } / 56$
C) $\sqrt { 113 } / 224$
D) $\sqrt { 113 } / 112$
E) $\sqrt { 113 } / 168$