$\frac{9}{2.5}$ + $\frac{9}{5.8}$ + $\frac{9}{8.11}$ + ... + $\frac{9}{29.32}$
= 3.$\frac{3}{2.5}$ + 3.$\frac{3}{5.8}$ + 3.$\frac{3}{8.11}$ + ... + 3.$\frac{3}{29.32}$
= 3.($\frac{1}{2}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{8}$ + $\frac{1}{8}$ - $\frac{1}{11}$ + ... + ... $\frac{1}{29}$ - $\frac{1}{32}$)
= 3.($\frac{1}{2}$ - $\frac{1}{32}$)
= 3.($\frac{16}{32}$ - $\frac{1}{32}$)
= 3.$\frac{15}{32}$
= $\frac{45}{32}$
mà $\frac{45}{32}$ = 1$\frac{13}{32}$
mà 1$\frac{13}{32}$ > 1
⇒$\frac{9}{2.5}$ + $\frac{9}{5.8}$ + $\frac{9}{8.11}$ + ... + $\frac{9}{29.32}$ > 1
