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Using Matlab for integration

  1. Dec 12, 2004 #1
    I used matlab to calculate the integration of a basic function.
    I wrote :

    F = inline('(2*x)+((x.^3)/0.000441)')

    Q = quad(F,0.01,0.05)

    I want to calculate integral of F from 0.01 to 0.05
    the answer shown is 0.059
    but the real answer is 0.361

    I am sure the function F is correct (I have tried some values).

    is there any problem with "Q = quad(F,0.01,0.05)" command???
    Thank you for your help...
     
  2. jcsd
  3. Dec 13, 2004 #2
    Why do you think the real answer is 0.361?
    I see that
    F(0.05) - F(0.01) = 0.361
    but this is of course not the integral, the integral is smaller.

    The function is increasing and so on the interval [0.01, 0.05] it is maximum at 0.05, there it is 0.3834. Overestimating the integral by assuming that F would be 0.3834 everywhere in the interval [0.01, 0.05] would give an area: 0.3834 * 0.04 = 0.0153
    which is close to 0.0059 (the answer that quad gives).
    I think 0.0059 is the right answer.
     
  4. Dec 13, 2004 #3

    ok now 1ve got it , 1 made a big mistake but 1 am happy to know my command is true . Thank you again
    :smile:
     
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