Lời giải:
Lời giải
$A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{18.19.20}$
$2.A=\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+....+\frac{1}{18.19}-\frac{1}{19.20}$
$2.A=\frac{1}{2}-\frac{1}{19.20}$
$2.A=\frac{1}{2}-\frac{1}{380}$
$<=>A=\frac{189}{380.2}$
$<=>A=\frac{189}{760}$