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Question
Math
Posted 9 months ago

Let φ(x,y)=x+2y\varphi(x, y)=x+2 \sqrt{y} and CC be the VV-shaped curve that starts at (1,4)(-1,4), makes a line to the origin, then makes a line to end at (1,4)(1,4)

Find the line integral of the gradient of φ\varphi around the curve CC.
Cφds=
\int_{C} \nabla \varphi \cdot d s=
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Answer from Sia
Posted 9 months ago
Solution by Steps
step 1
Calculate the gradient of the function φ(x,y)φ(x, y)
step 2
The gradient of φ(x,y)=x+2yφ(x, y) = x + 2\sqrt{y} is φ(x,y)=(φx,φy)=(1,1y)\nabla φ(x, y) = \left( \frac{\partial φ}{\partial x}, \frac{\partial φ}{\partial y} \right) = (1, \frac{1}{\sqrt{y}})
step 3
Break the line integral into two parts, one for each segment of the V-shaped curve
step 4
For the first segment from (1,4)(-1, 4) to (0,0)(0, 0), parameterize the curve as r1(t)=(1+t,44t)r_1(t) = (-1+t, 4-4t), where 0t10 \leq t \leq 1
step 5
Calculate dr1/dt=(1,4)dr_1/dt = (1, -4) and φ(r1(t))dr1/dt=(1,144t)(1,4)|\nabla φ(r_1(t)) \cdot dr_1/dt| = |(1, \frac{1}{\sqrt{4-4t}}) \cdot (1, -4)|
step 6
Integrate the dot product over the first segment: 01(1444t)dt\int_{0}^{1} (1 - \frac{4}{\sqrt{4-4t}}) dt
step 7
For the second segment from (0,0)(0, 0) to (1,4)(1, 4), parameterize the curve as r2(t)=(t,4t)r_2(t) = (t, 4t), where 0t10 \leq t \leq 1
step 8
Calculate dr2/dt=(1,4)dr_2/dt = (1, 4) and φ(r2(t))dr2/dt=(1,14t)(1,4)|\nabla φ(r_2(t)) \cdot dr_2/dt| = |(1, \frac{1}{\sqrt{4t}}) \cdot (1, 4)|
step 9
Integrate the dot product over the second segment: 01(1+44t)dt\int_{0}^{1} (1 + \frac{4}{\sqrt{4t}}) dt
step 10
Add the results of the integrals from steps 6 and 9 to find the total line integral around the curve CC
Answer
The line integral of the gradient of φφ around the curve CC is the sum of the integrals from steps 6 and 9.
Key Concept
Line Integral of a Gradient
Explanation
The line integral of the gradient of a scalar field along a curve is found by parameterizing the curve, computing the gradient, taking the dot product with the derivative of the parameterization, and integrating over the parameter interval.

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