The 180 K ? feedback resistor in the circuit in Fig. P5.14 is replaced by a variable res...

The 180 K ? feedback resistor in the circuit in Fig. P5.14 is replaced by a variable resistor R_f. The voltages v_a -v_d have the same values as given in Problem 5.14(a).
a) What value of R_f will cause the op amp to saturate? Note that 0 ≤ Rf ≤ ∞ .
b) When R_f has the value found in (a), what is the current (in microamperes) into the output terminal of the op amp?