1) So sánh \(A=\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{300^2}\) với \(\frac{3}{4}\).
2) Chứng minh rằng : \(B=\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+\frac{1}{6^2}+\frac{1}{7^2}+\frac{1}{8^2}<1\).
3) Chứng minh rằng : \(C=\frac{1}{10}+\frac{1}{11}+\frac{1}{12}+\frac{1}{13}+...+\frac{1}{99}>1\).
4) Tìm x, biết : 70 : \(\frac{4x+720}{x}=\frac{1}{2}\).