Engineering Using Thevenins therom to solve a simple circuits

[Apple&Orange]

Using Thevenins therom to solve a "simple" circuits

Homework Statement

The problems I am stuck on is problem two, figure 1 and figure 2. It's attached to this post.

Homework Equations

The Attempt at a Solution

My initial attempt at solving for R_thev for figure 1 was that R_thev=\frac{1}{4}+\frac{1}{14} = \frac{9}{28} = \frac{28}{9}.

However, on the answers it says that the 12K is shorted out since the source is connected at the same points, so it became R_thev=\frac{1}{4}+\frac{1}{2}=\frac{3}{4}=\frac{4}{3}. At first this made sense, but when I tried doing the second question, the 10K wasn't shorted out when solving for the R_thev.

Could someone please clarify why the 12K was shorted, but the 10K wasn't?

 

[NascentOxygen]

Could someone please clarify why the 12K was shorted, but the 10K wasn't?

In determining the Thevenin equivalent circuit,

(i) voltage sources are replaced by their internal series resistance, which for an ideal voltage source is zero ohms (i.e., a short circuit).

(ii) current sources are replaced by their internal series resistance, which for an ideal current source is infinite ohms (i.e., an open circuit).

Fig 2 involves a constant current source.

 

[Apple&Orange]

Got it, thanks a lot!