[tex] \frac{4}{7}(amplificam cu 4) - \frac{1}{28} = \frac{16}{28}- \frac{1}{28} = \frac{15}{28} [/tex]
[tex] \frac{9}{28} + \frac{1}{7} (amplificam cu 4)= \frac{9}{28}+ \frac{4}{28}= \frac{13}{28} [/tex]
[tex] \frac{15}{28} + \frac{13}{28} = \frac{28}{28} =1[/tex]
