Advanced Engineering Mathematics
Advanced Engineering Mathematics
10th Edition
ISBN: 9780470458365
Author: Erwin Kreyszig
Publisher: Wiley, John & Sons, Incorporated
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### Gradient and Directional Derivative Calculation

#### Problem 1: Finding the Gradient
Given the function:
\[ f(x, y) = 3x^3 - 4x^2y + y^2 \]
We are to find the gradient of \( f(x, y) \) at the point (1, -1).

#### Solution:
1. Compute the partial derivatives of \( f(x, y) \):
   - \(\frac{\partial f}{\partial x} = 9x^2 - 8xy \)
   - \(\frac{\partial f}{\partial y} = -4x^2 + 2y \)
   
2. Evaluate these partial derivatives at the point (1, -1):
   - \(\frac{\partial f}{\partial x}\bigg|_{(1, -1)} = 9(1)^2 - 8(1)(-1) = 9 + 8 = 17 \)
   - \(\frac{\partial f}{\partial y}\bigg|_{(1, -1)} = -4(1)^2 + 2(-1) = -4 - 2 = -6 \)
   
3. The gradient of \( f \) at (1, -1) is:
   \[ \nabla f(1, -1) = (17, -6) \]


#### Problem 2: Finding the Directional Derivative
Given the function:
\[ f(x, y) = 3x^2 - y^2 + 5xy \]
We are to find the directional derivative of \( f(x, y) \) at the point (2, -1) in the direction of the vector \(-3\mathbf{i} - 4\mathbf{j}\).

#### Solution:
1. Compute the partial derivatives of \( f(x, y) \):
   - \(\frac{\partial f}{\partial x} = 6x + 5y \)
   - \(\frac{\partial f}{\partial y} = -2y + 5x \)
   
2. Evaluate these partial derivatives at the point (2, -1):
   - \(\frac{\partial f}{\partial x}\bigg|_{(2, -1)} = 6(2) + 5(-1) = 12 - 5 =
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Transcribed Image Text:### Gradient and Directional Derivative Calculation #### Problem 1: Finding the Gradient Given the function: \[ f(x, y) = 3x^3 - 4x^2y + y^2 \] We are to find the gradient of \( f(x, y) \) at the point (1, -1). #### Solution: 1. Compute the partial derivatives of \( f(x, y) \): - \(\frac{\partial f}{\partial x} = 9x^2 - 8xy \) - \(\frac{\partial f}{\partial y} = -4x^2 + 2y \) 2. Evaluate these partial derivatives at the point (1, -1): - \(\frac{\partial f}{\partial x}\bigg|_{(1, -1)} = 9(1)^2 - 8(1)(-1) = 9 + 8 = 17 \) - \(\frac{\partial f}{\partial y}\bigg|_{(1, -1)} = -4(1)^2 + 2(-1) = -4 - 2 = -6 \) 3. The gradient of \( f \) at (1, -1) is: \[ \nabla f(1, -1) = (17, -6) \] #### Problem 2: Finding the Directional Derivative Given the function: \[ f(x, y) = 3x^2 - y^2 + 5xy \] We are to find the directional derivative of \( f(x, y) \) at the point (2, -1) in the direction of the vector \(-3\mathbf{i} - 4\mathbf{j}\). #### Solution: 1. Compute the partial derivatives of \( f(x, y) \): - \(\frac{\partial f}{\partial x} = 6x + 5y \) - \(\frac{\partial f}{\partial y} = -2y + 5x \) 2. Evaluate these partial derivatives at the point (2, -1): - \(\frac{\partial f}{\partial x}\bigg|_{(2, -1)} = 6(2) + 5(-1) = 12 - 5 =
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