Ta có: \(\frac{a}{b}< \frac{a+n}{b+n}\Leftrightarrow b\left(a+n\right)=a\left(b+n\right)=ab+an< ab+bn\)
\(\Leftrightarrow a< b\)( Vì n>0)
Tương tự :
\(\frac{a}{b}< \frac{a+n}{b+n}\Leftrightarrow a< b\)
\(\frac{a}{b}=\frac{a+n}{b+n}\Leftrightarrow a=b\)
Ta có: \(\frac{a}{b}< \frac{c}{d}\Rightarrow ad< bc\)
=> ad + ab < ab + bc
=> a(d + b) < b(a + c)
=> \(\frac{a}{b}< \frac{a+c}{b+d}\) (1)
Lại có: ad < bc
=> ad + cd < bc + cd
=> d(a + c) < c(b + d)
=> \(\frac{a+c}{b+d}< \frac{c}{d}\) (2)
Từ (1) và (2) => \(\frac{a}{b}< \frac{a+c}{b+d}< \frac{c}{d}\)