How is the following circuit solved
In the solution a supernode was formed between two nodes only
Where V1 : 280 volts

 

Where PL = Vth^2/4RL
RL = Rth
I just dont know how it is possible to form a supernode

 

How is the following circuit solved
In the solution a supernode was formed between two nodes only
Where V1 : 280 volts

Remember that nodal analysis is nothing more than the systematic application of KCL to the nodes in a circuit.

When we have a voltage source between two nodes, we don't have a way to determine the current that is flowing between them through the voltage source. But we DO have what we need to determine the voltage on one of the nodes in terms of the voltage on the other. So we take a box and put the two nodes into it and apply KCL to the lines leaving the box (which reduces the number of nodal equations for each additional node in the box) and we supplement those equations with equations relating the voltages on the nodes within the box (which brings us back up to the required number of equations overall).