a: Ta có: \(\frac13=\frac{1\times840}{3\times840}=\frac{840}{2520};\frac35=\frac{3\times504}{5\times504}=\frac{1512}{2520}\)
\(\frac45=\frac{4\times504}{5\times504}=\frac{2016}{2520};\frac57=\frac{5\times360}{7\times360}=\frac{1800}{2520}\) ;
\(\frac58=\frac{5\times315}{8\times315}=\frac{1575}{2520};\frac79=\frac{7\times280}{9\times280}=\frac{1960}{2520}\)
mà 840<1512<1575<1800<1960<2016
nên \(\frac13<\frac35<\frac58<\frac57<\frac79<\frac58\)
b: Ta có: \(\frac12=\frac{1\times30}{2\times30}=\frac{30}{60};\frac54=\frac{5\times15}{4\times15}=\frac{75}{60}\)
\(\frac95=\frac{9\times12}{5\times12}=\frac{108}{60};\frac23=\frac{2\times20}{3\times20}=\frac{40}{60}\)
\(\frac{13}{6}=\frac{13\times10}{6\times10}=\frac{130}{60};\frac{11}{5}=\frac{11\times12}{5\times12}=\frac{132}{60}\)
mà 30<40<75<108<130<132
nên \(\frac12<\frac23<\frac54<\frac95<\frac{13}{6}<\frac{11}{5}\)
