4. a)ƯCLN(4,10) 4=2² 10=2.5 =>ƯCLN(4,10)=2 BCNN(4,10) 4=2² 10=2.5 =>BCNN(4,10)=2².5=4.5=20 b)ƯCLN(300,280) 300=2².3.5² 280=2³.5.7 =>ƯCLN(300,280)=2².5=20 BCNN(300,280) 300=2².3.5² 280=2³.5.7 =>BCNN(300,280)=2³.3.5².7=8.3.25.7=4200 c)ƯCLN(30,90) 30=2.3.5 90=2.3².5 =>ƯCLN(30,90)=2.3.5=10.3=30 BCNN(30,90) 30=2.3.5 90=2.3².5 =>BCNN(30,90)=2.3².5=2.9.5=90 d)ƯCLN(14,21,56) 14=2.7 21=3.7 56=2³.7 =>ƯCLN(14,21,56)=7 BCNN(14,21,56) 14=2.7 21=3.7 56=2³.7 =>BCNN(14,21,56)=2³.3.7=8.3.7=168 5. a)ƯCLN(24,10) 24=2³.3 10=2.5 =>ƯCLN(24,10)=2 =>ƯC(24,10) THUỘC Ư(2)={1;2} =>ƯC(24,10)={1;2} b)ƯCLN(80,144) 80=2⁴.5 144=2⁴.3² =>ƯCLN(80,144)=2⁴=16 =>ƯC(80,144) THUỘC Ư(16)={1;2;4;8;16} =>ƯC(80,144)={1;2;4;8;16} c)ƯCLN(9,8,72) 9=3² 18=2.3² 72=2³.3² =>ƯCLN(9,18,72)=3²=9 =>ƯC(9,18,72) THUỘC Ư(9)={1;3;9} =>ƯC(9,18,72)={1;3;9} d)ƯCLN(25,55,75) 25=5.5 55=5.11 75=3.5² =>ƯCLN(25,55,75)=5 =>ƯC(25,55,75) THUỘC Ư{5}={1;5} =>ƯC(25,55,75)={1;5} 6. a)BCNN(10,24) 10=2.5 24=2³.3 =>BCNN(10,24)=2³.3.5=8.3.5=120 =>BC(10,24) THUỘC B{120}={0;120;240;360;480;....} =>BC(10,24)={0;120;240;360;480;...} b)BCNN(48,120) 48=2⁴.3 120=2³.3.5 =>BCNN(48,120)=2⁴.3.5=16.3.5=240 =>BC(48,120) THUỘC B{240}={0;240;480;720;...} =>BC(48,120)={0;240;480;720;...} c)BCNN(20,35,60) 20=2².5 35=5.7 60=2².3.5 =>BCNN(20,35,60)=2².3.5.7=420 =>BC(20,35,60) THUỘC B(420)={0;420;840;1260;...} =>BC(20,35,60)={0;420;840;1260;....} d)BCNN(18,24,32) 18=2.3² 24=2³.3 32=2⁵ =>BCNN(18,24,32)=2⁵.3²=32.9=288 =>BC(18,24,32) THUỘC B(288)={0;288;576;864;...} =>BC(18,24,32)={0;288;576;864;....} 7. a)89-(73-x)=20 73-x=89-20 73-x=69 x=73-69 x=4 b)(x+7)-25=13 x + 7=13+25 x+7=28 x=28+7 x=31 c)198-(x+4)=120 x+4=198-120 x+4=78 x=78-4 x=74 d) giống đề C e)140:(x-8)=7 x-8=140:7 x-8=20 x=20+8 x=28 f)4(x+41)=400 x+41=400:4 x+41=100 x=100-41 x=59