\(\eqalign{
& 20) \cr
& A.\,\,x \in \left( {{\pi \over 2};{{3\pi } \over 2}} \right) \Rightarrow x + \pi \in \left( {{{3\pi } \over 2};{{5\pi } \over 2}} \right) \cr
& \Rightarrow \sin \left( {x + \pi } \right)\,\,DB/\left( {{{3\pi } \over 2};{{5\pi } \over 2}} \right) \cr
& \Rightarrow - \sin \left( {x + \pi } \right)\,\,NB/\left( {{{3\pi } \over 2};{{5\pi } \over 2}} \right) \cr
& \Rightarrow Loai\,\,A. \cr
& C.\,\,x \in \left( {0;\pi } \right) \Rightarrow x + \pi \in \left( {\pi ;2\pi } \right) \cr
& \Rightarrow \sin \left( {x + \pi } \right)\,\,ko\,\,DB,NB\,\,/\left( {\pi ;2\pi } \right) \cr
& \Rightarrow Loai\,C \cr
& D.\,\,x \in \left( {\pi ;2\pi } \right) \Rightarrow x + \pi \in \left( {2\pi ;3\pi } \right) \cr
& \Rightarrow \sin \left( {x + \pi } \right)\,\,ko\,\,DB,NB\,\,/\left( {2\pi ;3\pi } \right) \cr
& \Rightarrow Loai\,\,D \cr
& Chon\,\,B. \cr
& 24) \cr
& x \in \left( {{{5\pi } \over 6};{{7\pi } \over 4}} \right) \cr
& BBT: \cr} \)
Vậy y=sinx nhận giá trị thuộc \(\left[ { - 1;{1 \over 2}} \right)\).
Chọn D.
