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\sqrt{\frac{0,3}{1,2}}=\sqrt{\frac{1}{4}}=\sqrt{\frac{1^2}{2^2}}=\frac{1}{2}
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\sqrt{1.4*2\frac{6}{7}}=\sqrt{\frac{14}{10}*\frac{20}{7}}=\sqrt{2*2}=\sqrt4=2
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\frac{1}{4}-0,6:(-\frac{1}{2})=\frac{1}{4}+\frac{6}{10}*\frac{2}{1}=\frac{1}{4}+\frac{6}{5}=\frac{6}{20}=\frac{3}{10}=0,3
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\frac{1}{5}:(1,75-2)=\frac{1}{5}:(-0,25)=\frac{1}{5}*(-\frac{1}{4})=-\frac{1}{20}