Find a vector u for which Duf(1,1) =0
Thank you.
well first for the definition of directional derivative:
directional derivative of a scalar function, in the direction of a unit vector, at a point is given by:
in your question:
at f(1,1):
therefore from;
now to obtain a zero and must be equal in magnitude but opposite in sign:
n can take any value.
Unit vector of
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