Using thevenin equivalent is very good way to simply the circuit, because we know that the load can't "tell" the different between 2 circuits. But the thevenin's origin is really troublesome. I really don't know how to prove the thevenin's theorem or what's state of it. Kirchhoff Law has proof itself (electron into and out of a node must be equal). Node and Mesh current method are base on that proof too. But thevenin's theorem is different. It's really really annoy me!!! Please tell me how to prove it or which origin lead to thevenin's theorem...