Thevenin's Theorem: It states that it is possible to simplify any linear circuit, no matter how complex, to an equivalent circuit with just a single voltage source. 

By varying i and measuring v both voltage source Vs and value of resistance Rs can be computed as

V = Vs+ i .Rs

Norton Theorem:  :

Animation of Norton Theorem can be found here:

http://www.wisc-online.com/objects/ViewObject.aspx?ID=DCE10004

The Theorem states that it is possible to simplify any linear circuit, no matter how complex, to an equivalent circuit with just a single current source. From above equation i can be computed.

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