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a:Cách 1: \(\frac{21}{35}=\frac{21:7}{35:7}=\frac35\)
\(\frac{12}{40}=\frac{12:4}{40:4}=\frac{3}{10}\)
mà \(\frac35>\frac{3}{10}\left(5<10\right)\)
nên \(\frac{21}{35}>\frac{12}{40}\)
Cách 2: \(\frac{21}{35}=\frac{21\times8}{35\times8}=\frac{168}{280}\)
\(\frac{12}{40}=\frac{12\times7}{40\times7}=\frac{84}{280}\)
mà 168>84
nên \(\frac{21}{35}>\frac{12}{40}\)
b: Cách 1: \(\frac{35}{49}=\frac{35:7}{49:7}=\frac57\)
\(\frac{40}{64}=\frac{40:8}{64:8}=\frac58\)
mà \(\frac57>\frac58\left(7<8\right)\)
nên \(\frac{35}{49}>\frac{40}{64}\)
Cách 2: \(\frac{35}{49}=\frac{35\times64}{49\times64}=\frac{2240}{3136}\)
\(\frac{40}{64}=\frac{40\times49}{64\times49}=\frac{1960}{3136}\)
mà 2240>1960
nên \(\frac{35}{49}>\frac{40}{64}\)
c: Cách 1: \(\frac{9}{11}=1-\frac{2}{11}\)
\(\frac{13}{15}=1-\frac{2}{15}\)
Ta có: 11<15
=>\(\frac{2}{11}>\frac{2}{15}\)
=>\(-\frac{2}{11}<-\frac{2}{15}\)
=>\(-\frac{2}{11}+1<-\frac{2}{15}+1\)
=>\(\frac{9}{11}<\frac{13}{15}\)
Cách 2: Ta có: \(\frac{9}{11}=\frac{9\times15}{11\times15}=\frac{135}{165}\)
\(\frac{13}{15}=\frac{13\times11}{15\times11}=\frac{143}{165}\)
mà 135<143
nên \(\frac{9}{11}<\frac{13}{15}\)
d: Cách 1: \(\frac{21}{7}=\frac{21\times5}{7\times5}=\frac{105}{35}\)
mà 105>24
nên \(\frac{21}{7}>\frac{24}{35}\)
cách 2: Ta có: \(\frac{21}{7}=3>1;1>\frac{24}{35}\)
Do đó: \(\frac{21}{7}>\frac{24}{35}\)
e: Cách 1: \(\frac12=\frac{1\times2424}{2\times2424}=\frac{2424}{4848}\)
Cách 2: \(\frac{2424}{4848}=\frac{2424:2424}{4848:2424}=\frac12\)
f: Vì 19>11
nên \(\frac{19}{15}>\frac{11}{15}\)
a) \(\frac{7}{8}=\frac{14}{16}>\frac{11}{16}\)
b) \(\frac{15}{35}=\frac{3}{7}=\frac{24}{56}\)
c) \(\frac{20}{31}>\frac{20}{33}>\frac{19}{33}\)
d) \(\frac{11}{12}<\frac{14}{12}=\frac{7}{6}\)
\(\dfrac{7}{15}và\dfrac{5}{13}=\dfrac{7.13}{15.13}=\dfrac{91}{195};\dfrac{5}{13}=\dfrac{5.15}{13.15}=\dfrac{75}{195}\)
\(\dfrac{11}{12}và\dfrac{7}{48}=\dfrac{11.4}{12.4}=\dfrac{44}{48};\dfrac{7}{48}\)
\(\dfrac{24}{35}và\dfrac{21}{7}=\dfrac{24}{35};\dfrac{21}{7}=\dfrac{21.5}{7.5}=\dfrac{105}{35}\)
\(\dfrac{15}{16}và\dfrac{7}{8}=\dfrac{15}{16};\dfrac{7}{8}=\dfrac{7.2}{8.2}=\dfrac{14}{16}\)
21/35=3/5; 12/20=3/5
=>21/35=12/20
24/35<1<21/7
35/49=5/7; 40/64=5/8
=>35/49>40/64
2424/4848=1/2
9/11=1-2/11
13/15=1-2/15
mà 2/11>2/15
nên 9/11<13/15
19/15>11/15
khó
ko bt lun