abhishek we cannot prove language is infinite since we have to wait for infinite time , hence we cannot say yes .
- if for any machine we can say yes only then semidecidable.
- if can say yes or no then decidable.
Same for finite since complete of it so undecidable proved by sice tyes =$\varnothing$ , tno = $\Sigma ^{*}$
Here tyes $\subseteq$ tno so not RE hance undecidable.