Với mọi n ,ta luôn có:
n+1<n+2 và n+3<n+5
Vì n+1<n+2.
=>\(\frac{n+1}{n+5}< \frac{n+2}{n+3}\)
Vì n+3<n+5.
=>\(\frac{n+2}{n+5}< \frac{n+2}{n+3}\)
=>\(\frac{n+1}{n+5}< \frac{n+2}{n+3}\)
Vậy....
Dựa vào tính chất : Nếu \(\frac{a}{b}< 1\)=) \(\frac{a}{b}< \frac{a+m}{b+m}\)( \(a,b\in N\))
Ta có : \(\frac{n+1}{n+5}< 1\)=) \(\frac{n+1}{n+5}< \frac{n+1+1}{n+5+1}=\frac{n+2}{n+6}< \frac{n+2}{n+3}\)
=) \(\frac{n+1}{n+5}< \frac{n+2}{n+3}\)