The superposition theorem is that the current in a branch of a linear circuit is equal to the sum of the currents produced by each source, with other sources set equal to zero.

The technique is to compute the sub-current based on each source, and then add them up to compute the total current. It is used in the next two theorems.

Thevenin’s theorem is to compute the Thevenin voltage and Thevenin resistance such that the circuit can be represented to an equivalent Thevenin circuit.

The technique of computing Thevenin voltage is to open up terminals of interest (A and B). Then the Thevenin resistance can be derived from all of the other resistances using Ohm’s law.

The technique of computing Thevenin resistance is to replace the dc source with a short, and compute the equivalent resistance between terminals of interest (A and B). The current and be derived then.

Norton’s theorem is very similar to Thevenin’s theorem. The difference is to compute the Norton current by shorting terminals of interest (A and B). By using the same technique mentioned above, we can compute the Norton resistance (which is the same to Thevenin resistance).