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Thread: integrate (1-2sin2x)/-cos2x dx

  1. #1
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    integrate (1-2sin2x)/-cos2x dx

    Hello could anyone integrate the following with respect to x

    (1-2sin2x)/(-cos2x) dx

    Could you please show the working as well.

    Thank you.
    Last edited by cocoboy; Oct 2nd 2017 at 09:53 PM.
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  2. #2
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    Re: integrate (1-2sin2x)/-cos2x dx

    Hey cocoboy.

    Hint - Have you considered a substitution integral [hint think about a suitable u and a du term]?
    Thanks from cocoboy
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  3. #3
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    Re: integrate (1-2sin2x)/-cos2x dx

    Why not just simplify the integrand?
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  4. #4
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    Re: integrate (1-2sin2x)/-cos2x dx

    The first thing I would do is multiply numerator and denominator by cos(2x). That gives me a "cosine" in the numerator that I can use with the "dx" and $cos^2$ in the numerator that can be changed to $1- sin^2(2x)$:
    $-\int \frac{1- 2sin(2x)}{cos(2x)}dx= -\int\frac{1- 2sin(2x)}{cos^2(2x)}(cos(2x)dx)= -\int\frac{1- 2sin(2x)}{1- sin^2(2x)}(cos(2x) dx)$

    Now let $u= sin(2x)$ so that $du= 2 cos(2x) dx$ and the integral becomes
    $-\frac{1}{2}\int \frac{1- 2u}{1- u^2}du$.
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  5. #5
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    Re: integrate (1-2sin2x)/-cos2x dx

    $$ \begin{aligned} \int \frac{1-2\sin {(2x)}}{-\cos {(2x)}} \, \mathrm dx &= - \int \sec{(2x)} - 2\tan{(2x)} \, \mathrm dx \\ &= -\bigg( \tfrac12 \ln{\big|\sec{(2x)} + \tan{(2x)}\big|} + \ln{\big|\cos{(2x)}\big|} \bigg) +c \\ &= - \bigg( \tfrac12 \ln{\big|\sec{(2x)} + \tan{(2x)}\big|} + \tfrac12 \ln{\big|\cos^2{(2x)}\big|} \bigg) +c \\[8pt] &= - \tfrac12 \ln{\big|\cos{(2x)} + \sin{(2x)}\cos{(2x)}\big|} +c \\[4pt] &= - \tfrac12 \ln{\bigg|\cos{(2x)}\big(1 + \sin{(2x)}\big)\bigg|} +c \end{aligned} $$
    The integrals of the two terms are ones that you should remember as standard.

    (For some reason, this wouldn't render under [tex][/tex] tags).
    Last edited by Archie; Oct 3rd 2017 at 05:55 AM.
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