Because, <a href="http://www.codecogs.com/eqnedit.php?latex=\int \frac{f'(x)}{f(x)} ~ dx = lnf(x) @plus; C" target="_blank"><img src="http://latex.codecogs.com/gif.latex?\int \frac{f'(x)}{f(x)} ~ dx = lnf(x) + C" title="\int \frac{f'(x)}{f(x)} ~ dx = lnf(x) + C" /></a>
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