Op amp gain calculations with resistor divider
Think of that voltage divider in terms of its equivalent Thevenin circuit. 







simulate this circuit &ndash; Schematic created using CircuitLab

Inside the box is the Thevenin equivalent circuit of your voltage divider. Where

$$ V_{\mathrm{TH}}=\frac{V_{\mathrm{cc}}R_1}{R_1+R_2}=2.5\mathrm{V}$$ and \$R_{\mathrm{TH}}=R_1||R_2\$

So, to the outside world that voltage divider looks like a voltage source with a series resistance, all you need to find is the value of the source and the value of that resistance. From there should be easy to see why you have those two resistors in parallel in your gain equation.