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Closed form integral of abs(cos(x))

  1. Jul 15, 2014 #1
    Hi everyone.

    Recently, I came across a closed form solution to ∫|cos(x)|dx as
    sin(x-∏*floor(x/∏+1/2)) + 2*floor(x/∏+1/2)

    I have no idea how to reach this solution but checking this for definite integral from 0 to 3∏/4 or ∏ seems to work. Using |cos(x)| as cos(x)*sgn(cos(x)) doesn't help in reaching at the solution. Does someone know how to get this closed form?

    Thanks.
     
  2. jcsd
  3. Jul 16, 2014 #2

    D H

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    Staff Emeritus
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    That is correct as a definite integral,
    [tex]
    \int_0^x \lvert \cos t \rvert \, dt =
    \sin\left(x - \pi \left\lfloor \frac x \pi + \frac 1 2\right\rfloor \right)
    + 2\left\lfloor \frac x \pi + \frac 1 2\right\rfloor
    [/tex]

    As an indefinite integral it's better to write [itex]\int \lvert\cos x\rvert\,dx = \sin x \operatorname{sgn}(\cos x)+C[/itex]

    How to get that closed form? By being creative. You're not going to find any of the standard integration methods that will yield that nice closed form solution.
     
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