Pregunta
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Part 1 of 8 A function \( f \) and a point \( P \) are given. Let \( \theta \) correspond to the direction of the directional derivative. Complete parts a. through \( e \). \( f(x, y)=\sqrt{3+2 x^{2}+2 y^{2}}, P(\sqrt{3},-1) \) a. Find the gradient and evaluate it at \( P \). The gradient at \( P \) is \( \langle\square, \square \). (Type exact answers, using radicals as needed.)

Ask by Harper Weber. in the United States
Mar 17,2025

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The gradient at point \( P(\sqrt{3}, -1) \) is \( \left\langle \frac{2\sqrt{3}}{\sqrt{11}},\, -\frac{2}{\sqrt{11}} \right\rangle \).

Solución

**Step 1. Write the function in a convenient form** We have \[ f(x,y)=\sqrt{3+2x^2+2y^2}=(3+2x^2+2y^2)^{\frac{1}{2}}. \] **Step 2. Find the general partial derivatives** We differentiate using the chain rule. For the partial derivative with respect to \(x\): \[ f_x(x,y)=\frac{1}{2}(3+2x^2+2y^2)^{-\frac{1}{2}}\cdot (4x)=\frac{2x}{\sqrt{3+2x^2+2y^2}}. \] For the partial derivative with respect to \(y\): \[ f_y(x,y)=\frac{1}{2}(3+2x^2+2y^2)^{-\frac{1}{2}}\cdot (4y)=\frac{2y}{\sqrt{3+2x^2+2y^2}}. \] Thus, the gradient of \(f\) is \[ \nabla f(x,y)=\left\langle\frac{2x}{\sqrt{3+2x^2+2y^2}},\,\frac{2y}{\sqrt{3+2x^2+2y^2}}\right\rangle. \] **Step 3. Evaluate the gradient at \(P(\sqrt{3}, -1)\)** Substitute \(x=\sqrt{3}\) and \(y=-1\) into the gradient formula. First, compute the value under the square root: \[ 3+2(\sqrt{3})^2+2(-1)^2=3+2(3)+2(1)=3+6+2=11. \] Then, \[ f_x(\sqrt{3}, -1)=\frac{2\sqrt{3}}{\sqrt{11}},\quad f_y(\sqrt{3}, -1)=\frac{2(-1)}{\sqrt{11}}=-\frac{2}{\sqrt{11}}. \] Thus, the gradient at \(P\) is \[ \nabla f(\sqrt{3}, -1)=\left\langle \frac{2\sqrt{3}}{\sqrt{11}},\, -\frac{2}{\sqrt{11}} \right\rangle. \] **Final Answer:** \[ \langle \frac{2\sqrt{3}}{\sqrt{11}},\, -\frac{2}{\sqrt{11}} \rangle. \]

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Bonus Knowledge

To find the gradient of the function \( f(x, y) = \sqrt{3 + 2x^2 + 2y^2} \), we first compute the partial derivatives of \( f \) with respect to \( x \) and \( y \). 1. **Partial derivative with respect to \( x \)**: Using the chain rule: \[ \frac{\partial f}{\partial x} = \frac{1}{2\sqrt{3 + 2x^2 + 2y^2}} \cdot (4x) = \frac{2x}{\sqrt{3 + 2x^2 + 2y^2}} \] 2. **Partial derivative with respect to \( y \)**: Similarly, \[ \frac{\partial f}{\partial y} = \frac{1}{2\sqrt{3 + 2x^2 + 2y^2}} \cdot (4y) = \frac{2y}{\sqrt{3 + 2x^2 + 2y^2}} \] Now, we evaluate these partial derivatives at the point \( P(\sqrt{3}, -1) \). First, we find \( 3 + 2(\sqrt{3})^2 + 2(-1)^2 \): \[ 3 + 2(3) + 2(1) = 3 + 6 + 2 = 11 \] So, \( \sqrt{3 + 2x^2 + 2y^2} \) at \( P \) is \( \sqrt{11} \). Now, substituting \( x = \sqrt{3} \) and \( y = -1 \): - For \( \frac{\partial f}{\partial x} \): \[ \frac{\partial f}{\partial x} \bigg|_P = \frac{2\sqrt{3}}{\sqrt{11}} \] - For \( \frac{\partial f}{\partial y} \): \[ \frac{\partial f}{\partial y} \bigg|_P = \frac{2(-1)}{\sqrt{11}} = \frac{-2}{\sqrt{11}} \] Thus, the gradient at \( P \) is: \[ \nabla f(P) = \left\langle \frac{2\sqrt{3}}{\sqrt{11}}, \frac{-2}{\sqrt{11}} \right\rangle \] So the gradient at \( P \) is \( \langle \frac{2\sqrt{3}}{\sqrt{11}}, \frac{-2}{\sqrt{11}} \rangle \).

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