đầu bài=$2(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{110})=$2(\frac{1}{3*4}+\frac{1}{4*5}+...+\frac{1}{10*11})=$2(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{10}-\frac{1}{11})=$2(\frac{1}{3}-\frac{1}{11})=$2*\frac{8}{33}=$\frac{16}{33}
đầu bài=$2(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{110})=$$2(\frac{1}{3*4}+\frac{1}{4*5}+...+\frac{1}{10*11})$=$2(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{10}-\frac{1}{11})=$$2(\frac{1}{3}-\frac{1}{11})$=$2*\frac{8}{33}=$$\frac{16}{33}$