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Thread: Confused about simple u substition problem for natural log

  1. March 16th 2010, 10:03 PM #1
    bfpri
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    Confused about simple u substition problem for natural log

    i'm trying to find the integral of 4/(6x+9). I let u be equal to 6x+9 and du be 6dx. Thus it is (4/6)lnu=(2/3)ln(6x+9). However my calculator says it is (2/3)ln(2x+3)...They are obviously different..What am i doing wrong?
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  2. March 16th 2010, 10:09 PM #2
    Anonymous1
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    Wait a minute... Are you sure thats what your calculator says? Because mine doesn't.

    http://integrals.wolfram.com/index.j...9&random=false
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  3. March 16th 2010, 10:29 PM #3
    bfpri
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    Yeah..thats what my TI-89 and my teacher says it is...
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  4. March 16th 2010, 10:42 PM #4
    redsoxfan325
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    Quote Originally Posted by bfpri View Post
    Yeah..thats what my TI-89 and my teacher says it is...
    There is no discrepancy. The two answers differ by a constant, so you can absorb it into the +C.

    \frac{2}{3}\ln(6x+9) = \frac{2}{3}\ln(2x+3)+\frac{2}{3}\ln(3)
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