Confusion about equivalent transformation of circuits through Thevenin's theorem

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Confusion about equivalent transformation of circuits through Thevenin's theorem [Copy link]

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The analysis is as follows:

I am confused about the equivalent transformation of the output circuit of the direct-coupled amplifier circuit using the Thevenin theorem. It is obviously connected in parallel with the transistor, so how is it equivalent to and in parallel? How does the equivalence come from (it looks like the result of series voltage division)? I am very confused and puzzled. I hope you can give me some advice. Thank you