In experiment 1, when you measured the output with the 51 ohm resistor load in place, you probably still noticed some loading effect. this is because an op amp can only output a certain amount of current before it begins behaving nonlinearly. every op amp has a maximum positive current, called "sourcing current", that it can generate to drive a load. (likewise there is also a maximum negative current, called "sinking current" that can flow into the output terminal of the op amp.) look up the lf412 spec sheet and find nominal value(s) for this sourcing current limit. based on this value, determine the minimum load resistance that could be driven by the lf412 op amp without experiencing any loading effects in experiment 1.