Transfrom scheme into eq. scheme with one EMF and one resistor.

[builder_user]

The new circuit is pic.1

What do I need to do with this two branches?OH!It's not R2.It's R3

 

[gneill]

One of the wires is redundant. You can't have less resistance than no resistance!
Remove one of them and solve for the net resistance across a-b.

 

[builder_user]

Rn=(R1*R3)/(R3+R1)

And new circuit is

 

[gneill]

Okay. Now put this Thevenin resistance in series with the Thevenin voltage source that you calculated before. That should be your final circuit, the Thevenin Equivalent for the original.

 

[builder_user]

The final circuit is...
E=U=1.6 V
Rn=0.875 Ohms

 

[gneill]

Yes. Be sure that you pay attention to the polarity of the voltage source. Remember that you found a negative value for E, making point a negative with respect to b.

 

[builder_user]

Remember that you found a negative value for E, making point a negative with respect to b.

I need to change the direction of E?
But E>0 because I3>0 & R3>0.

One of the tasks is done(I don't sure in this tense form)(another task is node potential method(only 2 nodes in the circuit so I try to make on my own(with rare success)).Thanks for your help.