Hi guys! Currently i've been learning on circuit principle particularly in Thevenin's theorem. I'm using Ralph J Smith's book but the explanation there is bit confusing. The steps to solved the problem is just hard to remember because it's jump over anything and i thing less systematic than what i found in All About Circuits. Usually, indeed i use Ralph J Smith's procedure. But the procedure is easy to forgotten. So, i search for easier method to remember and i found here.
But, i've found some trouble in here. There are one step that i don't know how to find, that is find R thevenin. Here is the screenshot :
I hope there are someone that will give explanation how to get 0,8Ohm resistor. Thanks.

 

R1//R3 = 4*1/(4+1) = 4/5 = 0.8

 

How can you study Thevenin's theorem if you don't know the basics ? This two resistors are connected in parallel, and everyone should know how to find equivalent resistance for this type of a circuit. This is really basic stuff. So you should start here :
https://en.wikipedia.org/wiki/Series_and_parallel_circuits#Resistors_2
http://www.allaboutcircuits.com/tex...chpt-5/what-are-series-and-parallel-circuits/
Or even here if you don't know the Ohm's law:
http://www.allaboutcircuits.com/textbook/direct-current/chpt-2/voltage-current-resistance-relate/