\(Câu.18:\\ 2x+4y< 5\\ \Leftrightarrow x+2y< \dfrac{5}{2}\Leftrightarrow x< \dfrac{5}{2}-2y\\ TH1:y=1\Rightarrow x< \dfrac{1}{2}\Rightarrow A.sai\left(Vì:1>\dfrac{1}{2}\right)\\ TH2:y=10\Rightarrow x< -\dfrac{35}{2}\Rightarrow B.sai\left(Vì:1>-\dfrac{35}{2}\right)\\ TH3:y=-1\Rightarrow x< \dfrac{9}{2}\Rightarrow C.đúng\left(Vì:1< \dfrac{9}{2}\right)\\ TH4:y=5\Rightarrow x< -\dfrac{15}{2}\Rightarrow D.sai\left(Vì:1>-\dfrac{15}{2}\right)\\ Chọn.C\)
\(Câu.16:\\ x-2+2\left(y-1\right)>2x+4\\ \Leftrightarrow x-2+2y-2>2x+4\\ \Leftrightarrow2y>2x-x+4+2-2\\ \Leftrightarrow2y>x+4\\ \Leftrightarrow y>\dfrac{1}{2}x+2\\ TH1:x=1\Rightarrow y>\dfrac{5}{2}\Rightarrow A.sai\left(Vì:1< \dfrac{5}{2}\right);B.đúng\left(Vì:5>\dfrac{5}{2}\right)\\ TH2:x=4\Rightarrow y>4\Rightarrow C.sai\left(Do:3< 4\right)\\ TH3:x=0\Rightarrow y>2\Rightarrow D.đúng\left(Do:4>2\right)\\ Vậy.Chọn:B,D\)
\(Câu.14:2x+y>1\Leftrightarrow y>1-2x\\ TH1:x=1\Rightarrow y>-1\Rightarrow A.đúng\left(Vì:1>-1\right)\\ TH2:x=2\Rightarrow y>-3\Rightarrow B.đúng\left(Vì:2>-3\right)\\ TH3:x=3\Rightarrow y>-5\Rightarrow C.đúng\left(Vì:3>-5\right)\\ TH4:x=-1\Rightarrow y>3\Rightarrow D.sai\left(Do:-1< 3\right)\\ Vậy.Chọn.D\)
\(Câu.12:\\ -3x+y+2\le0\Leftrightarrow y\le3x-2\\ TH1:x=1\Rightarrow y\le1\Rightarrow A.sai\left(Vì:2>1\right)\\ TH2:x=2\Rightarrow y\le4\Rightarrow B.đúng\left(Do:1< 4\right)\\ TH3:C.đúng\left(Do:\dfrac{1}{2}< 1\right)\\ TH4:x=3\Rightarrow y\le7\Rightarrow D.đúng\left(Do:1< 7\right)\\ Chọn.A\)