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Homework Help: Evaluate this definite integral 20x + 3x^2 - (2/3)x^3 from [-2, 5] ?

  1. Jun 16, 2010 #1
    1. The problem statement, all variables and given/known data

    I get the answer 19/3 but the real answer is 343/3. Can you show work to how it is that? I did this many times and still came up with 19/3 instead

    I got up to evaluating the integral part of finding the area between two curves but can never get the correct answer. I have a feeling I'm messing up signs somewhere.
  2. jcsd
  3. Jun 16, 2010 #2


    Staff: Mentor

    Are you sure you copied the problem correctly? I don't get either of your answers. It would be helpful for you to show what you did.
  4. Jun 16, 2010 #3

    Let f(x) = 20 + X + X2

    Let g(x) = X2-5X

    Find the area of the region enclosed by the two graphs.

    The graphs intersect at X = -2 and 5, so those are the bounds. F(x) is the upper curve.

    So I take the definite integral of

    [(20 + X - X^2) - (X^2-5X)dx] from [-2, 5]

    The answer is 343/3, but I do not get that answer when I compute the integral at all
  5. Jun 16, 2010 #4


    Staff: Mentor

    No, they intersect at only a single point, and it's not at either of those values of x.
    f(-2) = 20 -2 + 4 = 22
    g(-2) = 4 + 10 = 14

    f(5) = 20 + 5 + 25 = 50
    g(5) = 25 - 25 = 0

    If you set 20 + x + x2 = x2 - 5x, notice that you can subtract x2 from both sides, leaving you with a linear equation - an equation that has only one solution.

    I don't see that the two graphs enclose any region, so I don't know how you can do this problem. Are you sure you have all the information straight?
  6. Jun 16, 2010 #5


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    Staff Emeritus
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    Homework Helper

    The expression posted in the thread title contains an x3 term, so this isn't even the same problem.

    If you post the actual problem statement, exactly as it is written, that would be a big help.
  7. Jun 16, 2010 #6
    The problem in my Calc book just stated

    Let f(x) = 20 + X - X^2

    Let g(x) = X^2-5X

    Find the area of the region enclosed by the two graphs.

    I set these two equal to each other

    20 + X + X^2 = X^2-5X

    Through Algebra, I found that

    0 = 2X^2 - 6X -20

    which factors to


    Thus the curves intersect at X = -2 and X = 5. I made a mistake earlier saying that the graphs intersected at those two points; sorry.

    f(x) is the upper curve. From there they just used used the formula for calculating the area between curves and obtained the answer 343/3 but i cannot work out the math to get that
    Last edited: Jun 16, 2010
  8. Jun 16, 2010 #7


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    No, no, no! The two [itex]x^2[/itex] terms cancel they don't add!

  9. Jun 16, 2010 #8
    Aw damn, now I know why. Good catch. I wrote the equation for f(x) incorrectly. It was supposed to be

    20 + X - X^2 not 2+ + X + X^2

    Now what I wrote before makes sense
  10. Jun 17, 2010 #9
    If you have trouble getting the right anti-derivative then you can always try to compare your result with one from http://integrals.wolfram.com/index.jsp?expr=20x+++3x^2+-+(2/3)x^3&random=false
  11. Jun 17, 2010 #10


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    Staff Emeritus
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    Looks like you have set the integral up correctly. Can you show the rest of your work? I am getting 343/3 as well.
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