OP amp use as inverting amplifier

  Because no current flows into the input pins there can't be a voltage drop across ground connection.  Vin+ is therefore at 0V (this is called a virtual ground).  The output will adjust such that Vin- is at zero volts.  This makes Rin = R1.The current through R1 & R2 have to be the
same since no current goes into the input pins.Therefore I = Vin/R1.  Vout = Vin+ - IR2 =0 - (Vin/R1)R2.  Therefore Vout = -Vin(R2/R1)
The negative sine is because the current flows from the input to the output where as in the earlier examples the current flows from the output to the input.

Application hint: Why not connect Vin+ directly to ground? Actually, many people do and the circuit works fine. The reason to have R1||R2 is because real op-amps aren't perfect and draw a small bias current into both inputs. By adding R1||R2 the voltage drop at the + input is offset by the same amount as the voltage drop at the - input. This is called input offset voltage. edited by RDP