BEEE Verification of thevenin's theorem

Thevenin’s theorem states that a linear two-terminal circuit can be replaced by an equivalent circuit consisting of a voltage source VTh in series with a resistor RTh, where VTh is the open-circuit voltage at the terminals a and b when the respective load is removed and RTh is the input or equivalent resistance at the terminals when the independent sources are turned off. When the circuit is replaced by Thevevnin's equivalent across the load termials, then the load current can be obtained Iload=Vth/(Rth+Rload).