Answer:
gain in an op-amp is the output voltage divided by input voltage.
for the inverting amplifier , gain(Av)=-(Rf/Ri) ,where Rf => resistance of the feedback path & Ri=> inverting input terminal resistance
for the non-inverting amplifier , gain(Av)=(1+(Rf/Ri)) , where Rf => same & Ri=> non inverting input terminal resisance