To do this, we short-circuit the voltage source and open-circuit the current source. Then, $R_N$ is the equivalent resistance seen from terminals a-b.
$R_N = R_1 || R_2 = \frac{R_1 R_2}{R_1 + R_2} = \frac{12k\Omega \times 54k\Omega}{12k\Omega + 54k\Omega} =
Show more…