In a non-inverting op-amp, why is it R1/(R1+R2) instead of R2/(R1+R2) in this circuit?
If I redraw the circuit slightly...







simulate this circuit &ndash; Schematic created using CircuitLab

This is essentially what is around the OPAMP, a typical voltage divider circuit. V- is the voltage across R1 thus:

\$V- = V_o \frac{R1}{R1+R2}\$