Primitive of Reciprocal

Theorem

 * $\displaystyle \int \frac {\mathrm dx} x = \ln |x| + C$,

for $x \ne 0$.

Proof
Suppose $x > 0$.

Then this follows directly from derivative of the natural logarithm and the definition of indefinite integral.

Suppose $x < 0$.

Then:

And the result again follows from the definition of the indefinite integral.