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Math Help - help integrating

  1. #1
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    help integrating

    Hi im having trouble on where to start, im trying to itegrate sin(x)/cos^2(x). Should i make a u subsitution, if so for what?
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  2. #2
    Moo
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    Hello,

    Quote Originally Posted by cowboys111 View Post
    Hi im having trouble on where to start, im trying to itegrate sin(x)/cos^2(x). Should i make a u subsitution, if so for what?
    You can notice that \sin x is almost the derivative of \cos x.

    A direct substitution will be t=\cos x

    --> dt=-\sin x dx \implies dx=-\frac{1}{\sin x} dt

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  3. #3
    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by Moo View Post
    Hello,



    You can notice that \sin x is almost the derivative of \cos x.

    A direct substitution will be t=\cos x

    --> dt=-\sin x dx \implies dx=-\frac{1}{\sin x} dt

    You can also see that it can be written as : \frac{sin(x)}{cos(x)}\cdot \frac{1}{cos(x)} = sec(x)tan(x)

    Then,

    \int sec(x)tan(x) \,dx = sec(x)+C
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  4. #4
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    cool thank you!!
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  5. #5
    Moo
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    Quote Originally Posted by Chris L T521 View Post
    You can also see that it can be written as : \frac{sin(x)}{cos(x)}\cdot \frac{1}{cos(x)} = sec(x)tan(x)

    Then,

    \int sec(x)tan(x) \,dx = sec(x)+C
    Actually, I've never dealt with csc, sec, etc...
    What's the rule in here ?
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  6. #6
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    I have had several tell me that in Europe they do not deal with sec,csc,cot at all.

    Wonder why?.

    For your benefit. Their derivatives:

    \frac{d}{dx}[csc(x)]=-cot(x)csc(x)

    \frac{d}{dx}[cot(x)]=-csc^{2}(x)

    \frac{d}{dx}[sec(x)]=tan(x)sec(x)
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  7. #7
    Moo
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    Quote Originally Posted by galactus View Post
    I have had several tell me that in Europe they do not deal with sec,csc,cot at all.

    Wonder why?.
    In France, we've never seen such formulae... Only dealing with cos, tan, sin, and that was enough ^^ (in my high school...can't tell about all the others). Why ? I don't know, maybe because cot, csc, sec are known with sin, cos, tan... ~
    It's the same for the substitution. It seems that for you it's common to substitute, but here, we just recognize the derivative and apply formulae of antiderivatives we know



    Actually, by "here" in my previous post, I meant the equality of Chris where an antiderivative of sec*tan is sec..


    Edit : thanks a bunch !
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  8. #8
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    In my previous post, I went ahead and posted the derivatives of those functions.
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  9. #9
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    Here are their antiderivatives, Moo:

    \int{cot(x)}dx=ln(|sin(x)|)+C

    \int{sec(x)}dx=ln(|sec(x)+tan(x)|)+C

    \int{csc(x)}dx=ln(|csc(x)-cot(x)|)+C=ln(|tan(\frac{x}{2})|)+C
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  10. #10
    Moo
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    Really weird stuff

    Thanks
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  11. #11
    Eater of Worlds
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    Why ? I don't know, maybe because cot, csc, sec are known with sin, cos, tan... ~
    I reckon that's so. Same reason most calculators do not have them.
    One thing I noticed about calculators that do have them (such as the Voyage 200, TI-92, TI-89), they have the functions but only display in terms of sine and cosine. For instance, if you wanted the antiderivative of sec(x), the calculator displays it as ln(\frac{-cos(x)}{sin(x)-1}) instead of
    ln(sec(x)+tan(x))
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  12. #12
    Rhymes with Orange Chris L T521's Avatar
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    Quote Originally Posted by Moo View Post
    Actually, I've never dealt with csc, sec, etc...
    What's the rule in here ?
    Ah, I see that Galactus has explained it to you. They never taught that to you? At least you learned something new today!
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