If f=(1,4) at a given point P, a) Calculate the directional derivative of f(x…

If f=(1,4)\nabla f=\left(1,4\right) at a given point PP,

a) Calculate the directional derivative of f(x,y)f\left(x,y\right) at point PP in the direction of the vector 5,12\left\langle5,12\right\rangle.

b) Find a unit vector u\vec{u} such that the directional derivative of f(x,y)f\left(x,y\right) at point PP is zero.
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