Answer:
Homologous structures have the EXACT same side chains.
Ex) Are CH3 - CH2 - CH3 and CH3 - CH2 - CH2 - CH3 homologous or not?
These are homologus becasue neither of them have side chains and so there are no side chains to be compared. You don't need to look at the aprent chain itself. Thsoe are considered in Isomers.
Ex) Are CH3 - CH - CH3 and CH3 - CH - CH2 - CH2 - CH3 homologous or not?
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Br F
These two ( 2-bromopropane and 2-fluoropentane) are NOT homologous. You don't look at the parent chain itself but look at the side chains. For structures o be homologous, side chains have to be number the same too and so since bromine and fluorine are COMPLETELY different, the side chains are different and therefore this structure is not homologous.