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Math Help - Another confusing integration question

  1. #1
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    Another confusing integration question

    <br />
\int e^{2 x} \sin ( 6 x ) dx<br />
    I've tried subbing in both variables for u and dv using integration by parts, it only gets more complicated, I've also tried substitution, but that also leads to a dead end. Any suggestions?
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  2. #2
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    Use integration by parts TWICE and solve for the integral.

    It goes like this.

    1) Original Integral with sine (A) <== This is where we started.

    2) Expression with cosine (B) plus an integral with cosine (C). <== This is where one might give up, realizing that cosine and sine will just keep changing back and forth. Don't quit. Do it again.

    3) Expression with cosine (B) + expression with sine (D) + integral with sine (E). - Are you discouraged yet. You will be discouraged until you notice one thing, that "Integral with sine" is just "Original integral with sine" multiplied by a constant. I'll use a constant of (1/9).

    A = B + D + E

    A = B + D + (1/9)A

    (8/9)A = B + D

    A = (9/8)(B+D) and you are done.

    Definitely put this process in your pocket. It can be very useful. It should be interesting to you that the otherwise difficult calculus problem was solved by some rather ancient algebra. Don't forget your algebra!!
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  3. #3
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    Oh okay, Yeah when I did the integration by parts the first time noticed that cos and sin were flipping so I didn't bother. I'll try that, Thank you!
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  4. #4
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    Haha thanks for also using the constant 1/9, I was a little confused till I saw that while I was doing it.
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  5. #5
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    Ah, you found the lurking 1/9! Good work.
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  6. #6
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    there is a general rule :
    I=inverse
    L=Logarithmic
    A=algebraic
    T=trigonometric
    E=Exponwential
    Exponential functions are always to be integrated first then the trigonometric and so on..........i wonder whether that is applicable on this problem.
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