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Math Help - Confused about this trig substitution!

  1. #1
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    Confused about this trig substitution!

    intergral(tanx(secx)^2,x)

    my book tells me to make u = sec(x) du = tanxsecx dxwhich makes sense, but could you also make u = tanx making du = (secx)^2 dx? But the latter method is incorrect why?
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  2. #2
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    Quote Originally Posted by s3n4te View Post
    intergral(tanx(secx)^2,x)

    my book tells me to make u = sec(x) du = tanxsecx dxwhich makes sense,

    but could you also make u = tanx making du = (secx)^2 dx? Mr F says: Yes. And, in my opinion, this is by far the simplest approach to take.

    But the latter method is incorrect Mr F says: Says who!?

    why? Mr F says: It's not incorrect. In fact, it's better!
    ..
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  3. #3
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    Quote Originally Posted by s3n4te View Post
    intergral(tanx(secx)^2,x)

    my book tells me to make u = sec(x) du = tanxsecx dxwhich makes sense, but could you also make u = tanx making du = (secx)^2 dx? But the latter method is incorrect why?
    Where did it say that the second way was incorrect? You could use either.
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  4. #4
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    When I graphed both functions they aren't the same, is that normal?
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  5. #5
    Senior Member mollymcf2009's Avatar
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    Quote Originally Posted by s3n4te View Post
    When I graphed both functions they aren't the same, is that normal?
    The area under the curves will be the same which is what you are calculating using the integral.
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  6. #6
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    oh that make sense :P

    thanks
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