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Consider the vector field

F
and the curve C below.
F(x, y) = x4y5i + x5y4j,
C: r(t) =

t5 − 2t, t5 + 2t

, 0 ≤ t ≤ 1
(a) Find a potential function f such that
F
= ∇f

User Rouz
by
7.6k points

1 Answer

3 votes

A potential function for F is: f(x, y) = (x^5y^4)/4 + (x^5y^5)/20 + c

We know that a potential function exists for F if F is conservative. To check if F is conservative, we can use the curl test. The curl of F is given by:

curl F = ∂Q/∂x - ∂P/∂y

where P = x^4y^5 and Q = x^5y^4. Calculating the partial derivatives, we get:

∂Q/∂x = 5x^4y^4 - 4x^3y^5

∂P/∂y = 4x^4y^5 - 5x^5y^4

Setting these two equal, we get:

5x^4y^4 - 4x^3y^5 = 4x^4y^5 - 5x^5y^4

This simplifies to:

x^5y^4 - x^3y^5 = 0

Factoring out x^3y^4, we get:

x^3y^4(x^2 - 1) = 0

This means that F is conservative if x^3y^4 = 0 or x^2 - 1 = 0. We can see that x^3y^4 = 0 for all points on the curve C, so F is conservative.

Now, we can find a potential function f for F. We know that the gradient of f is equal to F, so:

∇f = F

This means that:

∂f/∂x = x^4y^5

∂f/∂y = x^5y^4

Integrating the first equation with respect to x, we get:

f(x, y) = ∫ x^4y^5 dx + g(y)

where g(y) is an arbitrary function of y. Integrating by substitution, we get:

f(x, y) = (x^5y^5)/5 + g(y)

Integrating the second equation with respect to y, we get:

f(x, y) = (x^5y^4)/4 + h(x)

where h(x) is an arbitrary function of x. Setting these two expressions for f(x, y) equal to each other, we get:

(x^5y^5)/5 + g(y) = (x^5y^4)/4 + h(x)

Solving for g(y), we get:

g(y) = (x^5y^4)/20 + c

where c is a constant of integration. Therefore, a potential function for F is:

f(x, y) = (x^5y^4)/4 + (x^5y^5)/20 + c

User Mestachs
by
7.9k points