1) Redraw, with load removed

2) Redraw, find Voc

3) Redraw, find Rth

4) Redraw, using TEC with removed load

Most of my questions involve finding Voc and Rth. I understand the steps, but I still do quite understand why certain things were done. Here is some examples:

(Prob 6 Questions) http://i37.tinypic.com/24nl5le.jpg

1) For Voc, they used KVL. Why did they use KVL for the perimeter and not the inner portion of the circuit?

Voc, perimeter: Voc + 5(0) - 20(4) - 35 = 0

2) When finding Rth, how do you specify which resistors contribute? What question do I need to be asking myself in order to determine Rth. For this problem I assume the the 3Ω resistor is negated due to it not being connected, i.e the resistor current cannot jump across the short.

(Prob 7 Questions) http://i37.tinypic.com/2ufr1x2.jpg

1) I understand how the 12Ω and 5Ω resistor have no current through them but how Vth determined? I don't understand what procedure or assumptions they made.

2) Why are only the 12Ω and 5Ω resistors contributing to Rth and not the 20Ω and 4Ω resistors as well?

(Prob 8 Questions) http://i35.tinypic.com/og038o.jpg

1) Again, what are the rules for determining Rth. Do you consider all directly connected resistors and indirectly connected resistors separated by a node?

Any help is appreciated, thanks!