Tính nhanh:a) $\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{18.19.20}$b) $\frac{5}{1.2.3}+\frac{5}{2.3.4?

"> Tính nhanh:a) $\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{18.19.20}$b) $\frac{5}{1.2.3}+\frac{5}{2.3.4?