A=$\frac{1}{99}$ +$\frac{12}{999}$ +$\frac{123}{9999}$ +$\frac{1234}{99999}$+ $\frac{+12345}{999999}$ .($\frac{1}{2}$ -$\frac{1}{3}$ -$\frac{1}{6}$ )
=$\frac{1}{99}$ +$\frac{12}{999}$ +$\frac{123}{9999}$ +$\frac{1234}{99999}$+ $\frac{+12345}{999999}$ .($\frac{3}{6}$- $\frac{2}{6}$- $\frac{1}{6}$ )
= $\frac{1}{99}$ +$\frac{12}{999}$ +$\frac{123}{9999}$ +$\frac{1234}{99999}$+ $\frac{+12345}{999999}$ .$\frac{0}{6}$
=$\frac{1}{99}$ +$\frac{12}{999}$ +$\frac{123}{9999}$ +$\frac{1234}{99999}$+ $\frac{+12345}{999999}$ .0
=0
Vậy: A=0
