Hello,
After you analyze the circuit you get something like this:
V2=-(k*Ri*Vs*R2)/(R1*R2+Ri*R2+k*Ri*R1+Ri*R1)
and to find out what happens when k gets infinite you can take the limit as k approaches plus infinity:
limit[k -->+inf](-(k*Ri*Vs*R2)/(R1*R2+Ri*R2+k*Ri*R1+Ri*R1))
and we get:
V2=-Vs*R2/R1
and note that now the response no longer depends on Ri as it did when k was finite. This is one of the main points of an op amp where we use a large gain to counter the effects of something else to get better accuracy overall. In practice k is not infinite but can be very very high like 100,000 or 1 million.