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Math Help - Greens Theorem Help

  1. #1
    Member Ranger SVO's Avatar
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    Greens Theorem Help

    The word document below should explain the problem
    Please help, so far your explinations have been better than the professors.
    The semester is almost over.

    Attached Files Attached Files
    Last edited by Ranger SVO; November 20th 2006 at 05:56 PM. Reason: Better explination
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  2. #2
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    What course is this
    (Calculus III does not teach this).

    The problem asks by Green's Theorem as opposed to the standard method of calculating the the line integral.

    You need to find,
    \oint_C 2xy\bold{i}+2x^2\bold{j} \cdot d\bold{R}
    Another notation for this (if that confuses you) is,
    \oint_C 2xydx+2x^2dy

    So the vector field is,
    \bold{F}(x,y)=3xy\bold{i}+2x^2\bold{j}
    Let,
    M(x,y)=3xy and N(x,y)=2x^2
    By Green's theorem,
    \int_D \int \left( \frac{\partial N}{\partial x}-\frac{\partial M}{\partial y} \right) \, dA
    Where,
    \frac{\partial N}{\partial x}=4x
    And,
    \frac{\partial M}{\partial y}=3x
    So the double integral over that closed region is,
    \int_D \int x\, dA

    Can you take it from there?
    (Hint: Divide your integral into two regions D_1,D_2 one is the square and the other is a semicircle.)
    ----
    (For Engineering Students)
    There happens to be a cool way to get that value if you ever taken an engineering course. You should recognize that is the first-moment. And hence you need to find the centroid of this shape. Which is simple for it is one of the standard shapes used.
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  3. #3
    Member Ranger SVO's Avatar
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    Ok, so why does he (the professor) have us parametrizing each of the paths and then intergrating them individually and adding them together.

    I am asking because what you have shown makes sense and its simpler.

    As always I thank you for taking time to answer my questions it is really appreciated.
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  4. #4
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    Quote Originally Posted by Ranger SVO View Post
    Ok, so why does he (the professor) have us parametrizing each of the paths and then intergrating them individually and adding them together.

    I am asking because what you have shown makes sense and its simpler.

    As always I thank you for taking time to answer my questions it is really appreciated.
    I think your professor wanted to do this problem in two ways. One, the standard way of parametrizing the simply connected positevly oriented piecewise smooth closed curve (this is one of my favorite expressions in math, just look how long it is and how cool it sounds). Second, by using Green's theorem because this is a simply connected positevly oriented piecewise smooth closed curve. And show that you get the same result in the end.
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