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An Area Problem

  1. Jun 20, 2005 #1
    I want to know how the area between two curves can be determined,do i just multiply the functions and then equate everything to 0,so i can get the limits,and the integrate the multiplied function within those limits
  2. jcsd
  3. Jun 20, 2005 #2


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    Mathelord, your description indicates some confusion. I plotted two functions:



    To find the area between them, in this particular case, you would subtract them:

    [tex]A=\int_0^2 [y2(x)-y1(x)] dx[/tex]

    [tex]=\int_0^2[(-x^2+4x)-x^2] dx[/tex]

    You can do the rest right?

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  4. Jun 20, 2005 #3


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    NO, you don't "multiply the functions" OR "equate everything to 0"! I wonder where you would have gotten the idea that you should multiply the two functions. The limits of integration are the values of x where the area "ends"- where the two curves intersect. To find where the curves y= f(x) and y= g(x) intersect, solve y= f(x)= g(x).

    Don't "integrate the multiplied function". Remember the "Riemann sums" that become the integral? Each term is the area of a skinny rectangle with width Δx and height the difference between the two functions: f(x)- g(x). You integrate the difference between the two functions.
  5. Jun 21, 2005 #4
    do i just subtract one from the other,which is the exact on to be subtracted from
  6. Jun 21, 2005 #5
    Subtract the lower function from the higher function.

    In Saltydog's example the lower function is x^2 and the upper function is 4x-x^2.
  7. Jun 21, 2005 #6
    in cases like ax^2+bx+c,and -ax^2-bx-c.which is the lower function so i can get one integrated
  8. Jun 21, 2005 #7
    Just graph them and check, or evaluate a test point, f(x) and g(x) to see which is lower.
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