How do I get the expansion of $(2x-1)^{-1} $ to the $x^3$ term How do I get the expansion of $(2x-1)^{-1}$ to the $x^3$ term
I don't really know how I have to use the formula here.
Do I have to use $$(1-2x)^{-1} = 1 + nx + \cdots$$

We know that 
$$(1-x)^{-1}=1+x+x^2+x^3+...$$
Then 
$$(2x-1)^{-1}=-(1-2x)^{-1}=-1-(2x)-(2x)^2-(2x)^3-...$$
