CONCEPT:Thevenin's Theorem:Any two terminal bilateral linear DC circuits can be replaced by an equivalent circuit consisting of a voltage SOURCE and a series resistor.To find Rth: To calculate Thevenin’s RESISTANCE we should replace all independent CURRENT SOURCES by Open circuit and Independent voltage sources by Short circuit (keep dependent sources as it is).Calculation:Independent sources will be short-circuited and the equivalent circuit is shown as:Since all resistances are connected in parallelThe equivalent Thevenin’s Resistance can be calculated as\(\frac{1}{{{R_{th}}}} = \frac{1}{4} + \frac{1}{4} + \frac{1}{4} = \frac{3}{4}\)Rth = 4 / 3 = 1.33 Ω