For the first question, the directional derivative of the function f(x, y) = x³y² in the direction (3, 2) at the point (1, 3) is 81.
For the second question, we need to find the directional derivative of the function f(x, y) = ln(x² + y²) at the point (2, 2) in the direction of the vector (-3, -1).
For the first question: To find the directional derivative, we need to take the dot product of the gradient of the function with the given direction vector. The gradient of f(x, y) = x³y² is given by ∇f = (∂f/∂x, ∂f/∂y).
Taking partial derivatives, we get:
∂f/∂x = 3x²y²
∂f/∂y = 2x³y
Evaluating these partial derivatives at the point (1, 3), we have:
∂f/∂x = 3(1²)(3²) = 27
∂f/∂y = 2(1³)(3) = 6
The direction vector (3, 2) has unit length, so we can use it directly. Taking the dot product of the gradient (∇f) and the direction vector (3, 2), we get:
Directional derivative = ∇f · (3, 2) = (27, 6) · (3, 2) = 81 + 12 = 93
Therefore, the directional derivative of f(x, y) in the direction (3, 2) at the point (1, 3) is 81.
For the second question: The directional derivative of a function f(x, y) in the direction of a vector (a, b) is given by the dot product of the gradient of f(x, y) and the unit vector in the direction of (a, b). In this case, the gradient of f(x, y) = ln(x² + y²) is given by ∇f = (∂f/∂x, ∂f/∂y).
Taking partial derivatives, we get:
∂f/∂x = 2x / (x² + y²)
∂f/∂y = 2y / (x² + y²)
Evaluating these partial derivatives at the point (2, 2), we have:
∂f/∂x = 2(2) / (2² + 2²) = 4 / 8 = 1/2
∂f/∂y = 2(2) / (2² + 2²) = 4 / 8 = 1/2
To find the unit vector in the direction of (-3, -1), we divide the vector by its magnitude:
Magnitude of (-3, -1) = √((-3)² + (-1)²) = √(9 + 1) = √10
Unit vector in the direction of (-3, -1) = (-3/√10, -1/√10)
Taking the dot product of the gradient (∇f) and the unit vector (-3/√10, -1/√10), we get:
Directional derivative = ∇f · (-3/√10, -1/√10) = (1/2, 1/2) · (-3/√10, -1/√10) = (-3/2√10) + (-1/2√10) = -4/2√10 = -2/√10
Therefore, the directional derivative of f(x, y) = ln(x² + y²) at the point (2, 2) in the direction of the vector (-3, -1) is -2/√10.
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