$\frac{1}{20.23}$+$\frac{1}{23.26}$+ $\frac{1}{y26.29}$+...+ $\frac{1}{77.80}$<$\frac{1}{9}$
Đặt A vào (1) ta có:
A=$\frac{1}{20.23}$+$\frac{1}{23.26}$+ $\frac{1}{y26.29}$+...+ $\frac{1}{77.80}$<$\frac{1}{9}$
Nhân 2 vé với 3 ta được:
3A=$\frac{3}{20.23}$+$\frac{3}{23.26}$+ $\frac{3}{y26.29}$+...+ $\frac{3}{77.80}$<$\frac{1}{9}$
3A=$\frac{1}{20}$-$\frac{1}{23}$+$\frac{1}{23}$-$\frac{1}{26}$+$\frac{1}{26}$-$\frac{1}{29}$+...+ $\frac{1}{77}$-$\frac{1}{80}$<$\frac{1}{9}$
3A=$\frac{1}{20}$-$\frac{1}{80}$<$\frac{1}{9}$
3A=$\frac{3}{80}$<$\frac{1}{9}$
⇒A=$\frac{1}{80}$<$\frac{1}{9}$
