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Anton [14]
3 years ago
10

For the vector field G? =(yexy+4cos(4x+y))i? +(xexy+cos(4x+y))j? G?=(yexy+4cos(4x+y))i?+(xexy+cos(4x+y))j?, find the line integr

al of G? G? along the curve CC from the origin along the xx-axis to the point (4,0)(4,0) and then counterclockwise around the circumference of the circle x2+y2=16x2+y2=16 to the point (4/2?,4/2?)(4/2,4/2).
Mathematics
1 answer:
gizmo_the_mogwai [7]3 years ago
4 0
\mathbf G(x,y)=(ye^{xy}+4\cos(4x+y))\,\mathbf i+(xe^{xy}+\cos(4x+y))\,\mathbf j

We're computing the line integral

\displaystyle\int_C\mathbf G\cdot\mathrm d\mathbf r

It looks like the circular part of C should be along the circle x^2+y^2=16 starting at (4,0) and terminating at \left(\dfrac4{\sqrt2},\dfrac4{\sqrt2}\right).

Because integrating with respect to a parameterization seems like it would be a pain, let's check to see if \mathbf G is a conservative vector field. For this to be the case, if \mathbf G(x,y)=P(x,y)\,\mathbf i+Q(x,y)\,\mathbf j, then \mathbf G is conservative iff \dfrac{\partial P(x,y)}{\partial y}=\dfrac{\partial Q(x,y)}{\partial x}.

We have P(x,y)=ye^{xy}+4\cos(4x+y) and Q(x,y)=xe^{xy}+\cos(4x+y). The corresponding partial derivatives are

\dfrac{\partial P(x,y)}{\partial y}=e^{xy}(1+xy)-4\sin(4x+y)
\dfrac{\partial Q(x,y)}{\partial x}=e^{xy}(1+xy)-4\sin(4x+y)

and so the vector field is indeed conservative.

Now, we want to find a function G(x,y) such that \nabla G(x,y)=\mathbf G(x,y)=\left(\dfrac{\partial G(x,y)}{\partial x},\dfrac{\partial G(x,y)}{\partial y}\right). We have

\dfrac{\partial G(x,y)}{\partial x}=ye^{xy}+4\cos(4x+y)

Integrating with respect to x yields

\displaystyle\int\frac{\partial G(x,y)}{\partial x}\,\mathrm dx=\int(ye^{xy}+4\cos(4x+y))\,\mathrm dx
G(x,y)=e^{xy}+\sin(4x+y)+g(y)

Differentiating with respect to y gives

\dfrac{\partial G(x,y)}{\partial y}=\dfrac{\partial}{\partial y}\left[e^{xy}+\sin(4x+y)+g(y)\right]
xe^{xy}+4\cos(4x+y)=xe^{xy}+\cos(4x+y)+\dfrac{\mathrm dg(y)}{\mathrm dy}
\implies \dfrac{\mathrm dg(y)}{\mathrm dy}=0
\implies g(y)=C

and so

G(x,y)=e^{xy}+\sin(4x+y)+C

Because \mathbf G(x,y) is conservative, and a potential function exists, the line integral is path-independent and the fundamental theorem of calculus of line integrals applies, so we can evaluate the line integral by evaluating the potential function at the endpoints. We end up with

\displaystyle\int_C\mathbf G\cdot\mathrm d\mathbf r=G\left(\frac4{\sqrt2},\frac4{\sqrt2}\right)-G(0,0)=e^8-1+\sin(10\sqrt2)
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Step-by-step explanation:

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