Could somebody explain to me why V- must equal V+ for an ideal op-amp?

 

Vout=A*(V1-V2)

where
A=differential gain
V1=voltage on the non-inverting input
V2=voltage on the inverting input
Vout=output voltage

Lets say the output is 10 volts and the gain is infinite

thus (V1-V2)=10/infinite=0

 

Makes sense. Thanks