Need help understanding the proof of Thevenin's theorem

[Pedro Cardoso]

Hello.
Uh, I'm trying to undestand how to prove Thevenin's theorem. The Sadiku book puts an independent current source where the load used to be in order to reach the equation: V = Vth + I*Rth. I do understand how he reaches that conclusion after putting the source, what I don't understand is why he can put that current source where the load was. Could someone explain to me that? I'm not sure if that is a formal proof, but it is a correct way of proving it, then I wish to learn it.

 

[Pedro Cardoso]

Nevermind, you prove it by using the substitution theorem...