I'm struggling to make a substitution in a symbolic expression where I want to use an approximation. For example if we have the equation
$$\frac{1}{a}\frac{c}{a - b} = \frac{c}{a^{2} - a b} \approx \frac{c}{a^{2}} \quad \text{as} \quad a \gg b $$
How can I do this symbolically with Mathematica. I have tried `Simplify[...]`, with

```
Simplify[(1/a)(c/(a^2 - ab)), a^2 - ab == a^2]

```

which unsurprisingly doesn't work. I also played around with

```
Simplify[(1/a) (c/(a - ab)), a \[GreaterGreater] b]

```

which doesn't seem to change things. I have also tried `Replace`

```
Replace[(1/a) (c/(a - b)), a^2 - a b -> a^2]

```

But again, no joy.

I have always struggle with symbolic substitutions, so if anyone can point to a good question on this site, or an external resource, I'd be much obliged.