a)\(\frac{5}{7}:X=1-\frac{4}{7}=\frac{3}{7}\)
\(X=\frac{5}{7}:\frac{3}{7}=\frac{5}{3}\)
b)
\(\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{14}+...+\frac{1}{19}-\frac{1}{21}\right)-X=\frac{4}{3}-\frac{221}{231}=\frac{29}{77}\)
\(\left(\frac{1}{11}-\frac{1}{21}\right)-X=\frac{29}{77}\)
\(X=\frac{10}{231}-\frac{29}{77}=-\frac{1}{3}\)
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