Bài 1: B={\(\frac{5}{-7};\frac{-7}{5};\frac{0}{5};\frac{0}{-7}\)}
Bài 2: Ta có: \(\frac{13}{17}=\frac{13.2}{17.2}=\frac{26}{34}\)
\(\frac{13}{17}=\frac{13.3}{17.3}=\frac{39}{51}\)
\(\frac{13}{17}=\frac{13.4}{17.4}=\frac{52}{68}\)
\(\frac{13}{17}=\frac{13.5}{17.5}=\frac{65}{85}\)