Cho \( a \in Z; b \in N*, n \in N* \) . Chứng minh rằng:
a) Nếu \( a < b \) thì \( \cfrac{ a}{b } < \cfrac{a+n }{ b+n} \)
b) Nếu \( a > b \) thì \( \cfrac{a }{ b} > \cfrac{ a+n }{ b+ n } \)
c) Nếu \( a = b \) thì \( \cfrac{ a}{ b} = \cfrac{ a+n }{ b+ n } \)