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1) 25 + 91 + 75 + 101
= (25+75) + (91 + 101)
= 100 + 192
= 292
2) 41 x 39 - 41 x 9
= 41 x (39 - 9)
= 41 x 30
= 1230
3) 710 x 99 + 710
= 710 x 99 + 710 x 1
= 710 x (99 + 1)
= 710 x 100
= 71000
4) 17 x 85 + 15 x 17 - 120
= 17 x (85 + 15) - 120
= 17 x 100 -120
= 1700 -120
= 1580
5) (39 x 42 - 37 x 42) : 42
= 42 x (39 - 37) : 42
= 42 x 2 : 42
= 84 : 42
=2
6) 120 - 48 - 52
= 120 - (48 + 52)
= 120 - 100
= 20
7) 23 x 75 + 25 x 23 + 180
= 23 x (75 + 25) +180
= 23 x (75 + 25) +180
= 23 x 100 + 180
= 2300 +180
=2480
8) 2448 : [119 - (23 - 6)]
= 2448 : (119 - 17)
= 2448 : 102
= 24
1/5+(1/20+1/21+1/22+1/23+1/24+1/25)+(1/101+1/102+103+104+105) Ta thấy 1/21;1/22;1/23;1/24;1/25 đều nhỏ hơn 1/20 nên 1/21+1/22+1/23+1/24+1/25<5×1/20<1/4 Tương tự 1/101+1/102+1/103+1/104+1/105<5×1/100<1/20 1/5+1/20+1/20=6/20=3/10 1/5+(<1/4)+(<1/20)<1/2 1/2=5/10 3/10<5/10 vậy suy ra điều cần chứng minh
1/5+(1/20+1/21+1/22+1/23+1/24+1/25)+(1/101+1/102+103+104+105)
Ta thấy 1/21;1/22;1/23;1/24;1/25 đều nhỏ hơn 1/20 nên
1/21+1/22+1/23+1/24+1/25<5×1/20<1/4
Tương tự
1/101+1/102+1/103+1/104+1/105<5×1/100<1/20
1/5+1/20+1/20=6/20=3/10
1/5+(<1/4)+(<1/20)<1/2
1/2=5/10
3/10<5/10 vậy suy ra điều cần chứng minh
\(\dfrac{9}{5}=\dfrac{18}{10}=1,8\)
\(\dfrac{2}{25}=\dfrac{8}{100}=0,08\)
\(\dfrac{23}{8}=\dfrac{2875}{1000}=2,875\)
\(\dfrac{76}{4}=\dfrac{1900}{100}=19\)
\(\dfrac{42}{25}=\dfrac{168}{100}=1,68\)
9/5 =18/10=1,8
2/25 =8/100=0,08
23/8 =2874/1000=2,874
76/4 =1900/100=19
42/25 =168/100=1,68
2/25=4/50=0,08
111/125=222/250=0,888
23/8=46/16=2,875 76/4=152/8=19 42/25=84/50=1,68 370/50=740/100=7,4
2/25=4/50=0,08
111/125=222/250=0,888
23/8=46/16=2,875 76/4=152/8=19 42/25=84/50=1,68 370/50=740/100=7,4
2/25=8/100=0,08
111/125=888/1000=0,888
23/8=2875/1000=2,875
76/4=1900/100=19
42/25=168/100=1,68
370/50=740/100=7,4
2/25=8/100=0,8
111/125=888/1000=0,888
23/8=2875/1000=2,875
76/4=1900/100=1,9
42/25=168/100=1,68
370/50=740/100=7,4
\(\frac{9}{5}=\frac{18}{10}=1,8\)
\(\frac{2}{25}=\frac{8}{100}=0,08\)
\(\frac{111}{125}=\frac{888}{1000}=0.888\)
\(\frac{23}{8}=\frac{2875}{1000}=2,875\)
\(\frac{76}{4}=\frac{1900}{100}=19\)
\(\frac{9}{5}=\frac{18}{10}=1,8\)
\(\frac{2}{25}=\frac{8}{100}=0,08\)
\(\frac{111}{125}=\frac{111.8}{125.8}=\frac{888}{1000}=0,888\)
\(\frac{76}{4}=19\)
\(\frac{23}{8}=\frac{23.125}{8.125}=\frac{2875}{1000}=2,875\)
\(\frac{42}{25}=\frac{42.4}{25.4}=\frac{168}{100}=1,68\)
\(\frac{370}{50}=\frac{37}{5}=\frac{37.2}{5.2}=\frac{74}{10}=7,4\)
71 4 17,75 31 30 20 2
23 8 2,875 70 60 40 0