3. Find the directional derivative, Daf(x, y), of the function f(x, y) = + y...

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3. Find the directional derivative, Daf(x, y), of the function f(x, y) = √² + y² at the point P(1,-1) in the direction from P(1,-1) to Q(4,-5). (10%) 5. Find the minimum value of f(x, y, z)= 3x² + 2y² + ² (10%) (10%) 3. Find the directional derivative, Daf(x, y), of the function f(x, y) = √² + y² at the point P(1,-1) in the direction from P(1,-1) to Q(4,-5). (10%) 5. Find the minimum value of f(x, y, z)= 3x² + 2y² + ² (10%) (10%)