I have an exam coming up on circuit theory & analysis,there's this one question I can't seem to get right.

The answers:

Vab=Vth=V(8-j15)=V(6+j10) this is what I used to get my Vth.

For Zth I used R(8-j15) // R(6+j10).

What am I doing wrong?

Textbook -- https://www.allaboutcircuits.com/textbook/direct-current/chpt-10/thevenins-theorem/

 

It appears you are assuming points A and B have the same voltage.

 

It appears you are assuming points A and B have the same voltage.

What do you mean?

 

Vab=Vth=V(8-j15)=V(6+j10)

Warning - it's after bar hours for me...

 

Hmm,thats the only way I could think of to get Vab,since R(8-j15) // R(6+j10).
What should the correct Vab be then?

 

You have 2 unknowns (A and B).

You know the source voltage across both series circuits that source both A and B.

Simultaneous equations or matrix solution is what I'm thinking.

 

You have 2 unknowns (A and B).

You know the source voltage across both series circuits that source both A and B.

Simultaneous equations or matrix solution is what I'm thinking.

You can use simultaneous equations?!?!
So far I've only learnt mesh analysis and the usual ohms law/kirchhoffs law so I'll have to get the answer using either one of those.

 

Solving the two equations is an algebraic step, not something that EE teaches, other than the required math courses for EE.

 

Here's the ebook review of mesh analysis ... http://www.allaboutcircuits.com/vol_1/chpt_10/3.html