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a) $0,(27)=0,27+0,0027+0,000027+...$
$\Rightarrow $ Tổng $\div\div$ lùi vô hạn có $q=0,01$; $S=\frac{0,27}{1-0,01}=\frac{27}{99} $
b) $0,21(354)=0,21+0,00354+0,00000354+...=0,21+\frac{0,00354}{1-0,001} $
$= 0,21+ \frac{0,00354}{0,999}=\frac{21}{100}+\frac{354}{999.100} = \frac{21(1000-1)+354}{99900} =\frac{21354}{99900}= \frac{10677}{49950}$
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