While reading a proof i came across this step which i could not understand. The chunk below is part of bigger expression, but in the interest of reducing noise i am just posting the sub expression if needed please let me know and i'll post the full expression.
$\left| \dfrac {1} {n\left( c+n-1\right) }\right| =\frac1{n^2}\left| 1-\dfrac {c-1} {n}+O\left( \dfrac {1} {n^{2}}\right) \right| $
What result is being used to accomplish this ?
Edit: $\left| \dfrac {\left( a+n-1\right) \left( b+n-1\right) } {n\left( c+n-1\right) }\right| = \left| 1+\dfrac {a-1} {n}\right| \left| 1+\dfrac {b-1} {n}\right|\left| 1-\dfrac {c-1} {n}+O\left( \dfrac {1} {n^{2}}\right) \right| $