Oblicz
a=\frac{5}{7}-\frac{2}{5}+\frac{1}{5}
a=\frac{5}{7}-\frac{2}{5}+\frac{1}{5}=
=\frac{5}{7}-\frac{1}{5}=
=\frac{25}{35}-\frac{7}{35}=\frac{18}{35}