1. $S=\frac{1}{50}+\frac{1}{121}+\frac{1}{182}+\frac{1}{243}+\frac{1}{601}+\frac{1}{662}+\frac{1}{723}$
So sánh S với $\frac{1}{20}$
2. $A=\left ( \frac{1}{1.2}+\frac{1}{3.4}+\frac{1}{5.6}+...+\frac{1}{99.100}+ \right ).51.52...99.100$
Chứng minh A chia hết cho 151