Summing Amplifier Zero Input Current/Voltage?
Why is the voltage at the inverting terminal of the input equal to zero?


Any opamp with negative feedback tries to keep its two inputs at the same voltage. Therefore, if the noninverting input is tied to ground, the action of the opamp is to keep the inverting input at ground, too.


  Why is the current at the inverting terminal of the input equal to zero?


An ideal opamp has infinite input resistance.