# Confused about simple u substition problem for natural log

• Mar 16th 2010, 10:03 PM
bfpri
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?
• Mar 16th 2010, 10:09 PM
Anonymous1
Wait a minute... Are you sure thats what your calculator says? Because mine doesn't.

http://integrals.wolfram.com/index.j...9&random=false
• Mar 16th 2010, 10:29 PM
bfpri
Yeah..thats what my TI-89 and my teacher says it is...
• Mar 16th 2010, 10:42 PM
redsoxfan325
Quote:

Originally Posted by bfpri
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 $+C$.

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