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Math Help - Calculus: Area Volume

  1. #1
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    Calculus: Area Volume

    1.
    Find the area enclosed between and , on the interval from to .
    Area =????

    2. Find the area of the region enclosed between and from to . Hint: Notice that this region consists of two parts.
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  2. #2
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    If f(x) \geq g(x) on the intervall I, then the area is \int_I (f(x)-g(x))dx

    The procedure is the same for the second problem, only that you you might need to split the integral into smaller intervals and switch the functions around (try to graph the functions to get a better overview).
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  3. #3
    Senior Member Pinkk's Avatar
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    Notice how f(x)>g(x) on [-3,7]. So the area is evaluated as:

    \int_{-3}^{7}\frac{x^{2}}{4}+10-x\,\,dx

    Simply evaluate the integral from there.


    For the next problem, find where the two graphs intersect and knowing how sine and cosine curves are on the given interval (which is [0,\frac{7\pi}{10}]), you'll know that to the left of the intersection 3cos(x)>2sin(x) and to the right of it 2sin(x)>3cos(x)

    3cos(x)=2sin(x)
    tan(x)=\frac{3}{2}
    x=tan^{-1}(\frac{3}{2})

    So the two intergrals needed are:

    \int_{0}^{tan^{-1}(\frac{3}{2})}3cos(x)-2sin(x)\,\,dx + \int_{tan^{-1}(\frac{3}{2})}^{\frac{7\pi}{10}}2sin(x)-3cos(x)\,\,dx
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  4. #4
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    So 1st in i plug everyting in and got 0. second one is 3.381389746??
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  5. #5
    Senior Member Pinkk's Avatar
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    The first one is definitely wrong. You can tell by the graph itself that it has to have an area greater than zero. Check your work again.
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  6. #6
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    According to your graph the first area is obviously not zero.

    EDIT: Beat me by seconds. ^_^
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  7. #7
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    So for the 1st one i plug in 7 and - the result when i plug in neg 3. I got 0

    ANyways thanks for your help..I give up.
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  8. #8
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    \int_{-3}^{7}\left(\frac{x^{2}}{4}+10-x\,\right)\,dx=\left[ \frac{x^3}{12} +10x - \frac{x^2}{2}\right]_{-3}^7 =\left[ \frac{x^3+120x-6x^2}{12}\right]_{-3}^7=\frac{889-(-441)}{12}=\frac{1330}{12}=\frac{665}{6}
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  9. #9
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    Oh i c i forgot the power rule..Thanks. for the second i just plug it in right?
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  10. #10
    Senior Member Pinkk's Avatar
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    What do you exactly mean by plug in? Evaluate the integrals and then evaluate them with the respective limits of integration.
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  11. #11
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    Thanks i got it 3.959622071.
    Quote Originally Posted by Pinkk View Post
    What do you exactly mean by plug in? Evaluate the integrals and then evaluate them with the respective limits of integration.
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