Use Theorem 13.9 to find the directional derivative of the function at P in the direction of the unit vector u = cos θi+ sin θj.

Data from in Theorem 13.9

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THEOREM 13.9 Directional Derivative If f is a differentiable function of x and y, then the directional derivative of fin the direction of the unit vector u = cos i + sin 0j is Duf(x, y) = f(x, y) cos 0 + f(x, y) sin 0.