Tutorial Exercise Use the gradient to find the directional derivative of the function at P in the direction of Q. f(x, y) = 3x² - y2 + 4, P(2, 4), Q(5, 9) Step 1 Since f is a polynomial, all the partial derivatives are continuous. Therefore, f is differentiable and you can apply the following theorem: if f is a differentiable function of x and y, then the directional derivative of f in the direction of the unit vector u is Duf(x, y) = Vf(x, y) · u. Find the given directional vector PQ so that we can find the unit vector. PQ = v - 2)i + (9 – 4)j i +

Calculus: Early Transcendentals
8th Edition
ISBN:9781285741550
Author:James Stewart
Publisher:James Stewart
Chapter1: Functions And Models
Section: Chapter Questions
Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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**Tutorial Exercise**

Use the gradient to find the directional derivative of the function at \( P \) in the direction of \( Q \).

\[ f(x, y) = 3x^2 - y^2 + 4, \quad P(2, 4), \quad Q(5, 9) \]

**Step 1**

Since \( f \) is a polynomial, all the partial derivatives are continuous. Therefore, \( f \) is differentiable and you can apply the following theorem: if \( f \) is a differentiable function of \( x \) and \( y \), then the directional derivative of \( f \) in the direction of the unit vector \( \mathbf{u} \) is 

\[ D_{\mathbf{u}}f(x, y) = \nabla f(x, y) \cdot \mathbf{u}. \]

Find the given directional vector \( \overrightarrow{PQ} \) so that we can find the unit vector.

\[ \overrightarrow{PQ} = \mathbf{v} \]

\[ = \left( \text{ } \_\_\_ \text{ } - 2 \right)\mathbf{i} + \left(9 - 4\right)\mathbf{j} \]

\[ = \text{ } \_\_\_ \mathbf{i} + \text{ } \_\_\_ \mathbf{j} \]
Transcribed Image Text:**Tutorial Exercise** Use the gradient to find the directional derivative of the function at \( P \) in the direction of \( Q \). \[ f(x, y) = 3x^2 - y^2 + 4, \quad P(2, 4), \quad Q(5, 9) \] **Step 1** Since \( f \) is a polynomial, all the partial derivatives are continuous. Therefore, \( f \) is differentiable and you can apply the following theorem: if \( f \) is a differentiable function of \( x \) and \( y \), then the directional derivative of \( f \) in the direction of the unit vector \( \mathbf{u} \) is \[ D_{\mathbf{u}}f(x, y) = \nabla f(x, y) \cdot \mathbf{u}. \] Find the given directional vector \( \overrightarrow{PQ} \) so that we can find the unit vector. \[ \overrightarrow{PQ} = \mathbf{v} \] \[ = \left( \text{ } \_\_\_ \text{ } - 2 \right)\mathbf{i} + \left(9 - 4\right)\mathbf{j} \] \[ = \text{ } \_\_\_ \mathbf{i} + \text{ } \_\_\_ \mathbf{j} \]
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