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Dido525

  • 2 years ago

Definition of the definite integral question:

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  1. Dido525
    • 2 years ago
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    Using the definition of the definite integral, compute: \[\int\limits_{-4}^{2}(3x^2+12x+20)dx\]

  2. malical
    • 2 years ago
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    Is the definition of a definition integral FTC or the definition of an integral: lim (x,y)-> (0,0) and on and on?

  3. jayz657
    • 2 years ago
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    take the anti derivative of 3x^2 + 12x + 20 is x^3 +6x + 20x then you plug in 2 and then -4 (2)^3 + 6(2) + 20(2) - [ -4^3 + 12(-4) + 20(-4) ] 8 + 12 + 40 - [ -64 - 48 - 80 ] 60 - [ - 192] = 252

  4. Dido525
    • 2 years ago
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    Definition of integral. @jayz657 Thanks but I want an explanation on this concept.

  5. Dido525
    • 2 years ago
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    I can calculate the anti derivative fine but I have to use Riemann sums @jayz657 .

  6. Dido525
    • 2 years ago
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    Just an explanation of some sort for Riemann sums would be fine. I can do the rest I am sure.

  7. jayz657
    • 2 years ago
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    |dw:1353040377471:dw| this is the riemann sum, you are drawing an infinite amount of rectangles under the curve here you know the length is change of x and the height is f(x) and you sum up each rectangle to get the area under the curve so will will get this \[\sum_{-4}^{2} f(x)\Delta x\]

  8. Dido525
    • 2 years ago
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    Yes I know the fundamental concept. But how would I exactly apply it?

  9. jayz657
    • 2 years ago
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    when you sum up the infinite amount of rectangles you will get the integral there

  10. malical
    • 2 years ago
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    You integrate the equation and then you get a function. That function will be in terms of f(x). The top number in the definite integral is b and the lower is a. f(b)-f(a) is your answer. Jayz got 252

  11. Dido525
    • 2 years ago
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    Yeah but How would I exactly sum up an infinite number of rectangles for my given function?

  12. malical
    • 2 years ago
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    Do you want to see how to do this problem or the theory behind it?

  13. Dido525
    • 2 years ago
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    @malical : I know how to find an antiderivative. I just have trouble applying riemann sums.

  14. malical
    • 2 years ago
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    Why are Riemann sums important?

  15. Dido525
    • 2 years ago
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    The question specifically says to use Riemann sums.

  16. Dido525
    • 2 years ago
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    Never mind. I got it :) .

  17. jayz657
    • 2 years ago
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    the change of x is the rate of change of each rectangle and rate of change is related to the derivative, dx so in this equation you can just draw as many rectangles as you want under the curve using a delta x width and using f(x) as your height heres an exmaple doing it the long way lets say i make 4 rectangles with length 3 |dw:1353040955063:dw|

  18. Dido525
    • 2 years ago
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    Yeah I got it thanks :) .

  19. jayz657
    • 2 years ago
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    ok np

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