Tìm tham số m để:
a, (-∞; 2m+1)⊂(-∞; 1)
b,(−∞;2−3m]∩[2;+∞)=∅(−∞;2−3m]∩[2;+∞)=∅
c, [−1;3]∩(2m−5;2m+4)=∅
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